A
A/B test
A controlled experiment comparing two versions of a page, ad or email to determine which produces a better result. Valid A/B testing requires a pre-declared sample size and duration; stopping a test early because one variant looks ahead is the most common source of false results.
ACoS
Advertising Cost of Sale. Ad spend divided by ad-attributed sales, expressed as a percentage. Used primarily on Amazon and other marketplaces. It ignores organic sales driven by advertising, which is why TACoS is usually the more informative metric.
Advantage+
Meta's automated campaign type in which audience selection, placement and creative combination are handled algorithmically rather than manually. It generally outperforms manual structures for ecommerce but requires a clean catalogue and steady creative supply.
AEO
Answer Engine Optimization. Optimising content to be selected as the direct answer in featured snippets, People Also Ask boxes and voice assistant responses. The unit that wins in AEO is the sentence rather than the page.
Affiliate marketing
A performance model where third-party partners promote a brand and earn commission on tracked sales. Its central discipline is incrementality — separating sales the partner created from sales that would have happened anyway.
AI Overviews
Google's generative summaries that appear above traditional search results, synthesising an answer from multiple sources with citations. Their growth has increased the share of searches that end without a click to any website.
Attribution
The method used to assign credit for a conversion across the touchpoints that preceded it. Common models include last-click, first-click, linear, time-decay and data-driven. All attribution is estimation; incrementality testing is the only way to establish causation.
Average order value (AOV)
Total revenue divided by number of orders. Raising AOV through bundling, cross-sell or minimum-spend thresholds improves the acquisition cost you can profitably afford.
B
Backlink
A link from another website to yours. Backlinks remain a significant ranking signal, but earned editorial links from relevant authoritative sites carry far more weight — and no penalty risk — than purchased ones.
Blended CAC
Total acquisition spend divided by total new customers, regardless of channel. Immune to attribution disputes, which is why it is more trustworthy than any single platform's reported cost per acquisition.
Bounce rate
The share of sessions with no further interaction. Largely superseded in GA4 by engagement rate. High bounce is not automatically bad — a visitor who found their answer immediately may bounce satisfied.
Broad match
A Google Ads keyword match type where the system serves ads for queries it judges related to your keyword. Effective when paired with smart bidding and disciplined negative keywords; expensive without them.
Buy Box
The purchase panel on an Amazon product page. Only one seller wins it at a time, and advertising is largely ineffective without it, making Buy Box percentage a prerequisite metric for marketplace advertising.
C
Cannibalisation
When two or more pages, campaigns or channels compete for the same demand, splitting signal and inflating cost. Common examples are multiple pages targeting one keyword, and Performance Max absorbing branded search.
CAPI
Conversions API. Meta's server-side event interface, which sends conversion data directly from your server rather than the browser. It recovers a meaningful share of conversions lost to tracking prevention and improves bidding quality.
Click-through rate (CTR)
Clicks divided by impressions. A diagnostic signal for ad and title relevance, not an outcome. A high CTR with poor conversion usually indicates a mismatch between promise and page.
Cohort analysis
Grouping customers by acquisition period and tracking their behaviour over time. Reveals whether newly acquired customers behave better or worse than earlier ones — often the earliest signal that growth is degrading.
Content decay
The gradual loss of rankings and traffic as content ages, competitors publish better versions and facts become outdated. Systematic refresh cycles typically return more per hour invested than new production.
Contribution margin
Revenue minus all variable costs including cost of goods, fulfilment, payment fees, returns and media. The number that actually funds a business, and the one most marketing reporting omits.
Conversion rate
The percentage of visitors who complete a desired action. Meaningful only when segmented by traffic source, since branded traffic converts several times better than cold prospecting.
Core Web Vitals
Google's page experience metrics: Largest Contentful Paint (loading), Interaction to Next Paint (responsiveness) and Cumulative Layout Shift (visual stability). They influence rankings and materially affect conversion rate.
Cost per acquisition (CPA)
Total cost divided by number of acquisitions. Useful only against gross margin — a CPA that is excellent in one business is ruinous in another.
Crawl budget
The resources a search engine allocates to crawling a site. Wasted on low-value or duplicate URLs, it delays discovery and indexing of pages that matter. Relevant mainly to large sites.
CRO
Conversion Rate Optimization. The systematic practice of increasing the share of visitors who convert, through research, hypothesis development and controlled experimentation. Its gains apply to traffic from every channel simultaneously.
D
Dayparting
Adjusting bids or budgets by time of day or day of week based on performance patterns. Useful where conversion behaviour varies predictably, such as B2B weekday patterns.
Deliverability
Whether email reaches the inbox rather than spam or promotions. Governed by authentication records (SPF, DKIM, DMARC), engagement rates, complaint rates and list hygiene.
E
E-E-A-T
Experience, Expertise, Authoritativeness and Trustworthiness. A framework describing what Google's systems and quality raters attempt to identify. Not a direct ranking factor, but it shapes what content succeeds — particularly in health, finance and legal categories.
Entity
A distinct thing — a company, person, product or concept — that search engines and language models recognise and connect to other entities. Consistent entity signals across the web are foundational to generative engine optimization.
Event match quality
Meta's score, from 1 to 10, of how well it can match your conversion events to real people. Scores below about 7 indicate substantial attribution loss and degraded optimisation.
F
First-party data
Data collected directly from your own customers and visitors with consent. Increasingly the foundation of targeting and measurement as third-party cookies and cross-site tracking decline.
G
GA4
Google Analytics 4, the current version of Google Analytics. Event-based rather than session-based, with a different data model and reporting approach from its predecessor Universal Analytics.
GEO
Generative Engine Optimization. Structuring content, data and entity signals so large language models cite your site when generating answers. Competes for inclusion in a synthesised answer rather than a ranked position.
Gross margin
Revenue minus cost of goods sold, as a percentage of revenue. Determines break-even ROAS and therefore what any advertising benchmark actually means for your business.
H
Holdout test
An experiment where a channel is switched off for a matched group — typically a set of geographic regions — to measure its true incremental contribution. The most reliable method available for proving causation.
Hook rate
In video advertising, three-second views divided by impressions. The strongest single predictor of video ad efficiency, because attention lost in the first three seconds cannot be recovered.
I
Incrementality
The additional outcome caused by a marketing activity, over and above what would have happened without it. Distinct from attributed conversions, which include outcomes that would have occurred anyway.
Internal linking
Links between pages on the same site. Shapes how search engines understand topic relationships and distributes authority toward commercial pages. One of the highest-leverage and least-used SEO levers.
J
JSON-LD
The recommended format for structured data markup. Placed in a script tag, it describes a page's content in machine-readable form for search engines and AI systems.
K
Keyword cannibalisation
Multiple pages on one site competing for the same keyword, splitting signals and typically causing all of them to rank worse than one consolidated page would.
L
Landing page
The page a visitor arrives at from a campaign. Message match between ad and page is among the strongest determinants of conversion rate and, in paid search, of quality score.
Lifetime value (LTV)
Total gross profit expected from a customer over their relationship with a business. Determines how much you can afford to spend acquiring them; commonly evaluated as a ratio against CAC.
llms.txt
An emerging convention: a plain-text or Markdown file at a domain root providing AI systems with a curated, machine-readable summary of a site's key pages and facts.
Lookalike audience
An audience built by a platform to resemble a source list of your existing customers. Value depends heavily on source quality — a lookalike of your highest-LTV customers behaves very differently from one built on all purchasers.
M
MER
Marketing Efficiency Ratio. Total revenue divided by total advertising spend. A blended, attribution-independent measure of whether marketing as a whole is working.
N
Negative keyword
A term excluded from triggering your ads. In accounts using broad match and smart bidding, the negative list does more to control spend than the keyword list does.
O
Organic traffic
Visits from unpaid search results. Distinguished from branded organic traffic, which largely reflects demand created elsewhere and should be analysed separately.
P
Performance Max
Google's automated campaign type spanning Search, Shopping, Display, YouTube, Discover, Gmail and Maps from a single campaign. Requires brand exclusions and feed quality to avoid absorbing traffic other campaigns would have won.
Programmatic advertising
Automated buying of ad inventory through real-time auctions across publisher networks. Offers scale and precision but requires careful fraud and brand-safety controls.
Q
Quality Score
Google Ads' estimate of ad and landing page relevance relative to competitors, rated 1 to 10. Higher scores lower cost per click for the same position.
R
Retargeting
Advertising to people who have previously interacted with your brand. Efficient by construction, and consequently a common source of inflated ROAS when its results are read as new customer acquisition.
RFM segmentation
Segmenting customers by Recency, Frequency and Monetary value. A durable and simple framework for lifecycle marketing, identifying which customers deserve which message.
Rich results
Enhanced search listings — review stars, FAQ expanders, product prices, breadcrumbs — generated from structured data. They increase visual prominence and click-through rate.
ROAS
Return on Ad Spend. Revenue divided by advertising cost. Meaningful only against gross margin; break-even ROAS equals one divided by gross margin.
S
Schema markup
Structured data using the Schema.org vocabulary that describes page content to machines. Foundational for rich results, for AI citation and for entity recognition.
SERP
Search Engine Results Page. Increasingly composed of AI overviews, shopping units, videos, People Also Ask boxes and forum results in addition to classic organic links.
Server-side tracking
Sending analytics and conversion events from your own server rather than the visitor's browser. Recovers data lost to tracking prevention and improves the quality of automated bidding.
Statistical significance
The confidence that an observed difference between test variants is not due to random chance. A 95 percent threshold is conventional; results called before reaching it are unreliable.
T
TACoS
Total Advertising Cost of Sale. Ad spend divided by total sales, both paid and organic. Falling TACoS alongside rising revenue indicates that advertising is successfully building organic rank.
Topical authority
The depth and breadth of a site's coverage of a subject area. Built through clusters of interlinked content rather than isolated pages, and increasingly important to both search rankings and AI citation.
U
UTM parameters
Tags appended to URLs to identify traffic source, medium and campaign in analytics. Consistent naming conventions are essential; inconsistent UTMs are among the most common causes of unusable reporting.
V
Value-based bidding
Bidding strategies that optimise toward conversion value rather than conversion count. Requires accurate value data — ideally gross margin rather than order value — to work as intended.
View-through conversion
A conversion recorded after someone saw but did not click an ad. Counted by some platforms and not others, and a frequent cause of divergence between platform-reported and actual results.
W
Whitelisting
Running paid ads from a creator's own social handle using their content, with their permission. Typically outperforms the same content run from a brand account because it retains social proof.
Z
Zero-click search
A search that ends without a click to any website, because the answer appeared directly in the results or an AI overview. A growing share of all searches, and the central strategic challenge of modern SEO.