A GA4 session meeting the applicable duration threshold, containing a key event, or including at least two page or screen views.
An engaged session in GA4 is a session that meets at least one of its engagement criteria. Under the default duration rule, it lasts longer than ten seconds; it can also qualify by containing a key event or at least two page or screen views. The web engagement timer can be configured.
The criteria are alternatives, not requirements that must all occur together. A short session with a key event can qualify, and a single-page session can qualify through duration. Conversely, the label does not prove that the visitor was satisfied or that a commercially useful action occurred.
Configuration matters when comparing reports. Changing the timer, key-event designations or collection behavior can change which sessions qualify. Document those changes so a movement in engaged sessions is not automatically attributed to better content.
Takeaway: Use engaged sessions as a defined behavioral measure, then assess the actual actions and customer outcomes that matter to the business.
Example
A visitor reads a single help page long enough to meet the configured duration rule and leaves. That can count as engaged without a form submission. Another visitor completes a properly measured key event quickly and can qualify by a different criterion.
What to check
Check the engagement timer and key-event setup before interpreting the metric. Test duplicate page views and event triggers, since faulty collection can distort classification. Do not describe every engaged session as a qualified lead.
For more detail, see GA4 engagement rate and bounce rate.