Engagement
Viewability
Also called viewable impression.
Viewability is whether an ad or element met a standard for time and pixels in view. The common standard is 50 percent of pixels for one second.
How it is measured
The script checks the element's visible area on scroll and on a timer. If the threshold holds, the impression is viewable. Video uses a longer rule.
Tarsier does not run ad viewability. It does not serve ads.
Worked example
A news site's leaderboard loads on 97 percent of pages but is viewable on 71 percent. The sidebar is viewable on 44 percent because it sits below the fold. Moving it up raises that to 63 percent.
The ad revenue per page grows 9 percent without a single extra view.
How it differs
Viewability is about pixels in view. An impression is delivery. A viewport is the window. Delivered does not mean seen.
Common errors
Mixing standards between vendors. Ignoring video rules. Counting lazy ads as viewable. Trusting one vendor's report. Forgetting mobile.
In practice
Pull viewability per slot and move or drop the weakest.