Digital advertising, defined and documented.
501 terms, 22 platform profiles, 117 vendors, current benchmarks and specs, comparisons, calculators, the standards that move the money, and the rules that constrain it. Written by people who buy media for a living. Free, and free of advertising.
Common lookups: CPM ROAS Incrementality Demand-Side Platform Header Bidding Conversions API Retail Media Viewability
Glossary
Every term in the trade, each on its own page, with related terms linked. Grouped by what it is used for.
- Buying and pricing (48)
- Measurement (100)
- Targeting and data (39)
- Creative and formats (81)
- Infrastructure and standards (78)
- Search (36)
- Practice and industry (84)
- Privacy and regulation (35)
Platforms
What each platform sells, how it is bought and measured, and what changed lately.
Benchmarks
CPM, CPC, CTR, conversion rate, and cost per lead ranges by channel, with the sources behind each number. Reviewed September 17, 2026.
Directory
117 companies across DSPs, SSPs, verification, identity, consent, CTV, retail media, and workflow, each in one neutral line. Nobody pays to be listed.
Ad specs
Dimensions, durations, and text limits for every major platform and channel, on one page.
Comparisons
18 side-by-side answers: DSP vs SSP, MMM vs attribution, PMax vs Advantage+, CTV vs linear, and more.
Calculators
CPM, CPA, ROAS, breakeven, reach and frequency, CAC to LTV, pacing, and lift test sample size.
Questions
29 answer-first pages: what is a good CTR, CPM, ROAS, or CPA; how much each platform costs; how to calculate the metrics.
Templates
Media plan, naming convention, launch QA checklist, RFP questions, and measurement plan. On the page and as CSV, no signup.
Standards
OpenRTB, VAST, ads.txt, sellers.json, TCF, GPP, the MRC viewability definition, and the rest of the plumbing, with links to the specifications.
Rules
The laws, regulations, and platform policies that decide what can be targeted, claimed, and shown, cross-referenced to the maintainer's regulation registers.
Questions this library gets asked most
What is the difference between CPM, CPC, and CPA?
CPM prices impressions (per thousand), CPC prices clicks, and CPA prices conversions. The further down that list, the more risk the seller carries and the higher the effective price. Most social and display media is billed on CPM regardless of what the campaign optimizes toward.
Which platforms are walled gardens?
Google, Meta, Amazon, TikTok, LinkedIn, Microsoft, Apple, Pinterest, Snap, Reddit, X, and Spotify each sell their own inventory with their own tools and keep user-level data inside. The open web is everything transacted through DSPs and exchanges outside them.
Why does platform-reported ROAS overstate results?
Each platform credits itself for conversions within windows it sets, including view-through conversions, and cannot see other platforms' touches. Adding platform ROAS figures together produces more conversions than the business recorded. Incrementality tests and marketing mix models measure what advertising caused.
Are third-party cookies gone?
Safari and Firefox block them by default. Chrome, after years of delays, abandoned plans to remove them in 2025 and kept them with user controls. Mobile in-app tracking on iOS is opt-in under App Tracking Transparency and most users decline.
What does a DSP cost?
Platform fees run roughly 10 to 20 percent of media on open-auction buys, plus data fees (often $0.50 to $3 CPM per segment), verification fees, and any agency fee. The 2023 ANA study found about a third of open-web programmatic spend never reached publishers.
What is the minimum budget to advertise on each platform?
Google, Microsoft, Meta, X, and Reddit have no meaningful minimum; TikTok requires about $50 per day per campaign; LinkedIn $10 per day; Snapchat and Pinterest a few dollars. Enterprise DSPs and Amazon DSP managed service carry annual or monthly commitments.
What counts as a viewable impression?
Under the MRC standard, at least half of the ad's pixels in view for at least one continuous second for display, two seconds for video. Viewability is an opportunity to see, not evidence that anyone looked.
Is retargeting worth it?
It reliably shows the best reported ROAS and reliably shows the least incremental lift when tested, because it reaches people already likely to convert. Run it capped, exclude recent converters, and measure with a holdout.
How this library is written
Entries state what a thing is, how it is priced or measured, and what a practitioner needs to know to use it or argue about it. Where the industry disagrees, the entry says so. Where a number changes often, the entry gives a range and a source rather than a false precision. Vendors are described, never rated, and no vendor has paid for a listing. Corrections with a source go to info@agencyechelon.com and are reviewed by a person.
The library is maintained by the Agency Echelon team alongside its registers of AI regulation, crypto regulation, and data privacy law. Text is released under CC BY 4.0 so it can be quoted, taught from, and cited.