Retention rate
Retention is the percentage of a video the average viewer completes, graphed over its runtime. The curve shows exactly where audiences leave — which is why sponsor-slot position is a negotiation point: a mention at 80% duration plays to a fraction of the viewers a 10% placement gets. Ask for the retention curve, not the average.
Worked example
Average retention 45% on an 8-minute video: an integration at minute one reaches most viewers; the same integration at minute six reaches a third.
Related: Watch time · CTR
Full context in the guides, or measure it yourself with the free tools.