Digital marketing glossary

Definitions with the formula attached, because a metric you cannot calculate is a metric you cannot argue with.

Two ways in below: look up one term fast, or start with the twelve that matter most.

Which metric applies when

The most common mistake is not misdefining a metric — it is reporting the right metric at the wrong stage. Impressions answer a question nobody is asking if the problem is that traffic does not convert.

1. 1

   Getting seen

   Is anyone seeing us?

   • Impressions
   • Reach
   • CPM

   None of these prove interest. Treat them as a denominator, not a result.

2. 2

   Getting attention

   Is the promise compelling?

   • Clicks
   • CTR
   • CPC

   A high CTR with a low conversion rate usually means the promise oversold the page.

3. 3

   Getting action

   Is the traffic the right traffic?

   • Conversions
   • Conversion rate
   • CPL

   Always state the denominator — per session, per user and per click differ noticeably.

4. 4

   Getting paid

   Did it make money?

   • CPA
   • CAC
   • ROAS
   • ROI

   This is the only row a finance team recognises. Report blended figures here.

Terms people confuse

The obscure words are rarely the problem. These six pairs cause most of the disagreements.

Money metrics

The numbers that decide whether marketing paid for itself. If you learn one group, learn this one.

Bounce rate

The share of sessions where the visitor did not take any further tracked action. What counts as a bounce depends entirely on what you chose to track, so it is easy to make it look better without improving anything.

Formula Single-action sessions / Total sessions

A page answering a question completely may show a high bounce rate and still be doing its job perfectly.

Click-through rate CTR Start here

The share of people who clicked after seeing your listing or ad. It measures how compelling the promise is, not how good the page behind it turns out to be.

Formula Clicks / Impressions

A search result shown 10,000 times and clicked 320 times has a 3.2% CTR.

Conversion rate Start here

The share of visits that complete the action you defined as valuable. Always state the denominator — conversion rate per session, per user and per click give noticeably different numbers.

Formula Conversions / Sessions

168 conversions from 8,420 sessions is a 2.0% conversion rate.

Cost per acquisition CPA Start here

What you paid in media for one conversion. Unlike CAC it usually excludes salaries and overheads, so it flatters the true cost of acquiring a customer.

Formula Spend / Conversions

£4,200 of spend producing 112 conversions gives a CPA of £37.50.

Cost per lead CPL

What you paid for one lead, before anyone has judged whether the lead is any good. Track it alongside lead-to-customer rate, or you will optimise towards cheap leads that never buy.

Formula Spend / Leads

Customer acquisition cost CAC Start here

The full cost of winning one new customer, including media, salaries, tools and agency fees — not just ad spend. CAC is the figure a finance team recognises; CPA usually is not.

Formula Total sales and marketing cost / New customers acquired

£20,000 of total marketing cost producing 50 new customers gives a CAC of £400.

Impressions

The number of times your listing or ad was displayed. An impression means it was served, not that anyone read it.

Return on ad spend ROAS Start here

Revenue produced per unit of media spend, expressed as a multiple. Per-platform ROAS double counts when several platforms claim the same sale, so blended ROAS is the figure that matches the bank account.

Formula Revenue / Ad spend

£13,440 of revenue from £4,200 of spend is a ROAS of 3.2x.

Return on investment ROI Start here

Profit relative to total cost, expressed as a percentage. ROI accounts for what the thing cost you to deliver; ROAS does not, which is why a campaign can show strong ROAS and negative ROI.

Formula (Profit − Cost) / Cost

Advertising

Cost per click CPC Start here

The average amount you pay each time someone clicks your ad. A rising CPC is not automatically bad — it often accompanies more competitive, higher-intent traffic.

Formula Spend / Clicks

Cost per mille CPM

The cost of a thousand ad impressions. Useful for comparing the price of reach between placements, and largely useless for judging whether a campaign worked.

Formula (Spend / Impressions) × 1,000

Pay per click PPC Start here

Advertising where you pay only when someone clicks, rather than for the impression. Most search advertising works this way.

Remarketing

Advertising to people who have already interacted with you. Google's term for the practice; functionally the same as retargeting, and the two names are used interchangeably in most conversations.

Retargeting

Showing ads to people who visited your site or engaged with your content but did not convert. Cheap to run and easy to overdo — frequency caps exist for a reason.

Measurement and analytics

These describe how the numbers are produced. They matter because a metric is only as trustworthy as the convention behind it.

Attribution Start here

The rules that decide which channel gets credit for a conversion when someone touched several before converting. Attribution is a choice, not a fact — change the model and the same data credits different channels.

A visitor arrives from a search ad, returns via email a week later, then converts. Last-click credits email; first-click credits paid search; a data-driven model splits it.

Key performance indicator KPI Start here

The small number of metrics you have agreed actually indicate success. If everything is a KPI, nothing is — most teams need one primary metric and two or three supporting ones.

Primary: qualified enquiries. Supporting: cost per enquiry, conversion rate, organic sessions.

Session

A group of interactions by one visitor within a time window. Sessions are a measurement convention rather than a real thing, and different analytics platforms end them under different rules.

UTM parameters UTM Start here

Tags added to a URL so analytics can attribute the visit to a specific source, medium and campaign. They only work if everyone follows one written convention — inconsistent capitalisation alone will split one campaign into several.

?utm_source=newsletter&utm_medium=email&utm_campaign=spring-launch

Channels

Landing page

The page someone arrives on from a campaign. A good one matches the promise made in the ad or listing exactly; mismatch between the two is one of the most common and most expensive paid media faults.

Search engine marketing SEM

Marketing through search engines. In current usage it usually means the paid side specifically, though it historically covered both paid and organic search.

Search engine optimisation SEO Start here

The practice of earning visibility in unpaid search results, by making pages that answer a query better than the alternatives and that search engines can crawl and understand.

Search engine results page SERP

The page of results returned for a query. Reading the actual SERP before targeting a keyword tells you what intent the engine believes the query has — which matters more than the keyword's volume.

Social media marketing SMM

Marketing through social platforms, organic or paid. The main strategic question is not which platform, but what job you expect it to do — reach, proof, support or direct response.

Strategy

Funnel

A model of the stages someone passes through from first awareness to purchase. Useful for deciding what a page is for; misleading if you expect real buyers to move through it in a straight line.

A comparison page sits near the bottom of the funnel; a definition page sits near the top.

Lead magnet

Something useful offered in exchange for contact details. The exchange only works when the thing offered is genuinely worth the email address — a thin PDF trains people to unsubscribe.

How to quote a metric so nobody can argue with it

Four habits that remove almost all ambiguity from a report:

1. State the denominator. “2.1% conversion rate” means nothing on its own. Per session, per user and per click give visibly different numbers from the same data.
2. Give the comparison. A number with no previous period and no target cannot be judged, so a reader will invent their own baseline.
3. Say whether it is blended. For anything involving spend and revenue, say explicitly whether the figure is per-platform or blended. This single word prevents the most expensive misunderstanding in paid media reporting.
4. Name the source. Analytics and the ad platform will disagree. Saying which one you used turns a contradiction into a footnote instead of a credibility problem.

Why this is one page

Individual “X meaning in digital marketing” pages are easy to produce and mostly not worth reading. A two-sentence definition on its own URL answers less than a glossary entry sitting next to the terms it gets confused with — and the confusion is the actual problem worth solving.

Keeping it as one page is what makes the comparison table above possible. CPA is genuinely easier to understand alongside CAC than in isolation.

If a specific term later proves to need more room than an entry allows — a full worked example, a calculator, a decision framework — it will get its own page then, on the evidence rather than in advance.