Session watch time
Also called: session time, watch session, session duration
Google's ranking work has looked at attention accumulated across a chain of videos in a single visit rather than the time banked on one watch page. That is the idea the phrase points at.
No figure for it exists in YouTube Analytics, so any session watch time a creator quotes for their own channel came from somewhere other than Studio. The useful part is what it implies about sending viewers onward. YouTube dropped its old rule reserving the watch page's first suggested slots for the channel being watched after its data showed unrestricted slots raised total watch time and viewer satisfaction, which makes pointing someone at a genuinely better video a reasonable move even when it is not yours.