Paid Media and AI Search Glossary

Each definition starts with what the term means for your money, then how it works.

A

ACoS (advertising cost of sale)

Amazon's main efficiency measure: ad spend divided by the sales those ads produced, as a percentage. An ACoS of 25% means every £1 of ad-driven sales cost 25p in advertising. Calculator. Link

Ad Rank

How Google decides whether an ad shows and in what position. It weighs the bid against the quality of the ad and landing page and the context of the search, which is why a more relevant ad can outrank a higher bid. Link

AEO (answer engine optimisation)

Shaping content so a search engine can lift it as the direct answer to a question, in a featured answer, an AI Overview or a voice reply. Guide. Link

AI Overviews

Google's AI-written summaries shown above some search results, with links to the pages they draw on. Link

Attribution

The rule that decides which ad, click or channel gets the credit for a sale. Every attribution figure is an inference, because a customer who saw three ads cannot tell you which one persuaded them. Link

Auto-tagging

A Google Ads setting that adds a click identifier (the GCLID) to the landing page address on every ad click, so a later conversion can be matched to that click. Link

B

Break-even ROAS

The return on ad spend at which advertising pays for itself and no more. It equals one divided by your profit margin: at a 25% margin, you need £4 of sales for every £1 of ads to break even. Calculator. Link

Broad match

The loosest Google Ads keyword match type. Ads can show for searches related to the keyword's meaning, not only the words in it, which widens reach and needs watching in the search terms report. Link

C

Click-through rate (CTR)

The share of people who saw an ad and clicked it. A low CTR on a search ad usually means the ad does not match what people searched for. Link

Consent mode

Google's way of passing a visitor's cookie choice to its tags, so they behave accordingly. Version 2 added two advertising signals, ad_user_data and ad_personalization. Guide. Link

Conversion

Whatever an ad platform is told counts as success: an enquiry, a booking, a sale. The platform spends your budget chasing it, so a wrongly defined conversion wastes money however good the campaign looks. Link

Conversion rate

The share of clicks, or visits, that become a conversion. Halving it doubles what each conversion costs, at the same cost per click. Link

Conversions API

Meta's route for sending conversions from your server or CRM rather than from the visitor's browser, so outcomes the browser misses still reach Meta. Link

Cost per acquisition (CPA)

Ad spend divided by the number of conversions. Also called cost per action, or cost per conversion. Link

Cost per click (CPC)

What you pay each time someone clicks an ad. On Google Search it is set in an auction for each search and is often below the maximum you bid. UK figures, 2026. Link

Cost per lead (CPL)

Ad spend divided by the number of enquiries. The useful version is the most you can afford per lead, worked out from what a customer is worth and how many leads become customers. Calculator. Link

CPM

Cost per thousand impressions: what it costs for an ad to be shown a thousand times. Meta usually charges this way. Link

Customer Match

Google's feature for uploading a list of your own customers, so you can reach them, exclude them or find people like them. It depends on consent for the data uploaded. Link

E

Enhanced conversions

A Google Ads feature that sends a hashed copy of first-party data, such as the email address a customer typed, with each conversion, so Google can match more conversions to the ads that drove them. Link

Exact match

The tightest Google Ads keyword match type. Ads show for searches with the same meaning as the keyword, which gives the most control and the least reach. Link

G

GCLID

Google Click Identifier: the code auto-tagging adds to the landing page address. Stored with an enquiry, it lets a sale made weeks later be imported back to Google Ads against the right click. Link

GEO (generative engine optimisation)

Making a business easy for AI systems such as ChatGPT, Gemini and Perplexity to understand, trust and name when they write an answer. Guide. Link

I

Impression share

The share of the auctions your ad was eligible for in which it actually showed. Impression share lost to budget means the money ran out; lost to rank means the ad or bid was not strong enough. Link

Incrementality

The sales an ad caused that would not have happened without it. It is the only measure of whether advertising adds anything, and it needs a test, such as switching spend off in one area, to measure. Link

L

Landing page

The page an ad click arrives on. Google weighs its relevance and experience in Ad Rank, and it decides most of the conversion rate. Link

llms.txt

A proposed plain-text file at a website's root that summarises the site for language models. Whether AI systems read it is unmeasured. Adoption measured, 2026. Link

M

MER (marketing efficiency ratio)

Total revenue divided by total marketing spend, across every channel. It ignores attribution entirely, which makes it a useful check on platforms that each claim the same sale. Link

N

Negative keyword

A word or phrase that stops an ad showing for searches that contain it, such as "free" or "jobs" for a business that sells a paid service. Link

O

Offline conversion import

Sending sales that happen after the click, on the phone or weeks later, back to the ad platform against the stored click ID, so bidding learns from real sales rather than form fills. Link

P

Performance Max

A Google Ads campaign type that runs across Google's channels, including Search, YouTube, Display, Gmail, Discover and Maps, from one campaign, with Google choosing where each ad shows. Link

Phrase match

The middle Google Ads keyword match type. Ads show for searches that include the meaning of the keyword, with more control than broad match and more reach than exact. Link

POAS (profit on ad spend)

Gross profit from ad-driven sales divided by ad spend. Unlike ROAS it accounts for what the products cost you, so a POAS above 1 means the ads made money before overheads. Calculator. Link

Q

Quality Score

Google's 1 to 10 rating of a keyword's expected click-through rate, ad relevance and landing page experience. It is a diagnostic, not a direct input to the auction. Link

R

Remarketing

Showing ads to people who have already visited your site or app. It needs the visitor's consent for advertising cookies in the UK. Link

ROAS (return on ad spend)

Revenue from ads divided by ad spend. A ROAS of 4, often written 400%, means £4 of sales for every £1 of ads. Whether that is profitable depends on your margin. Calculator. Link

S

Search terms report

The list of actual searches that triggered your ads. It is where wasted spend shows up, and where negative keywords come from. Link

Server-side tagging

Running tracking tags on a server you control instead of in the visitor's browser, which gives more control over what data is sent. It does not replace consent. Link

Smart Bidding

Google's automated bid strategies, such as target CPA and target ROAS, which set a bid for each auction. They optimise toward whatever conversions they are fed, so they are only as good as the tracking. Link

Structured data

Code on a page, usually JSON-LD, that tells machines what the page is about in a fixed vocabulary: who the business is, what it sells, who wrote the content. How many sites use it, 2026. Link

T

TACoS (total advertising cost of sale)

Amazon ad spend divided by all sales, including sales that did not come from ads. A falling TACoS while ad spend holds suggests ads are lifting sales beyond the ones they are credited with, which is an inference. Calculator. Link

Top-of-page bid

Google Keyword Planner's estimate of what advertisers have historically bid to show at the top of the results for a keyword. The low figure approximates the 20th percentile of those bids, the high figure the 80th. UK figures, 2026. Link

U

UTM parameters

Tags added to a link (utm_source, utm_medium, utm_campaign) so analytics can tell which campaign a visit came from. Google Ads does this itself through auto-tagging. Link