That's what we language nerds would call a damn good question. 😉
I'd say a session is a meet-up with the explicit purpose of doing something pre-planned. But in the video store days, my friends and I used to called them "shoots," so I guess for us, a shoot is a session that gets filmed. More recently, I've been involved with what everyone called "demos," which I suppose were just unrecorded large group sessions. But meet-ups for one-off tickling or other unfilmed play are definitely sessions in my head. So...they're all sessions, just with some being shoots, others being demos, and the rest unspecified.
With my wife it's more spontaneous, so I wouldn't call it a session. It's just...us.