This guide is designed to stand on its own as an educational reference. It separates technical concepts, commercial outcomes and policy considerations so readers can reason about the system rather than memorize terminology.
Served does not mean seen
An impression can be counted when an ad is delivered even if the user never scrolls far enough to bring it into view. Advertisers care about the opportunity for a person to notice the creative, not merely the fact that a server returned it. Viewability adds a visibility layer to impression measurement.
Eligible, measurable and viewable
Google's Active View terminology separates eligible impressions, measurable impressions and viewable impressions. Eligible means the impression can participate in measurement. Measurable means the technology successfully measured it. Viewable means it met the applicable threshold.
Why placement matters
Ads far below the fold may load long before a user reaches them. Sticky or in-content placements behave differently. Slow rendering can also reduce the chance that an ad becomes visible before the user leaves. Improving viewability involves page layout, loading strategy and user behavior.
Viewability is not attention
A viewable ad had an opportunity to be seen under a measurement rule. It does not prove that the user paid attention, understood the message or intended to act. Treat viewability as a quality signal, not a complete measure of human attention.
How to use the metric
Compare viewability by placement, device, page type and demand source. Pair it with latency, revenue and user-experience measures. A placement that raises viewability but degrades usability may not create long-term value.
A diagnostic example
Suppose a placement has high fill but low viewability. Ads are being served, but users may rarely see them. If measurability is also low, there may be a technical measurement issue. Segmenting eligible, measurable and viewable events prevents different problems from being collapsed into one score.