Watch time is the total amount of content viewers actually watch, aggregated across users and titles. It is the headline engagement number for most services because it captures real usage better than plays or logins: a play that stops after thirty seconds and one watched to the end are very different, and watch time reflects that.
It anchors a family of metrics - watch time per user, per title, per session - and feeds both content decisions (what is actually being watched) and infrastructure planning (watch time at a given bitrate drives egress cost). For ad-supported services more watch time means more ad inventory; for subscriptions it is a leading indicator of the engagement that precedes retention.
