Engagement
Engaged time
Also called active engaged time.
Engaged time is time the page was visible and receiving periodic signs of activity. It is stricter than visibility alone.
How it is measured
A heartbeat fires every few seconds while the tab is in front and the person has scrolled, clicked or typed recently. Each heartbeat adds seconds. Idle stretches stop the clock.
Tarsier sends a ping every 15 seconds while the tab is visible and one more when the page is hidden. That is visibility-based. If you need input checks, you have to add them with your own events.
Worked example
A tutorial page has its video playing on a second monitor. Visible time reads 22 minutes. Engaged time reads 4 minutes, because the viewer touched nothing for most of the run.
The author concludes people read the written steps for four minutes and then let the video run on its own.
How it differs
Engaged time requires activity. Engagement time requires only visibility. An attention metric is a vendor-built score. Choose the simplest of the three that answers your question.
Common errors
Mixing visible time and engaged time in one chart. Ignoring that reading uses no input. Setting idle limits too short. Scoring video watching as inactive. Forgetting assistive tools that produce few events.
In practice
If you use engaged time, set the idle limit to about 30 seconds and apply it only to pages that work the same way.