Session
A session is a single continuous visit by one user, bundling all their activity, page views, clicks, and events, until they leave or go inactive long enough to time out (often 30 minutes). It's the basic unit most analytics tools use to count visits, so metrics like sessions per user, pages per session, and session duration all hang off this definition.
For a founder, the thing to know is that a session is a convention, not a fact. The same person across two days is two sessions; leave a tab open over lunch and one visit can split into two. That means session counts inflate or deflate depending on the timeout rule and how you handle returning users. Use sessions for engagement trends, but lean on user-level and cohort metrics for anything you'll make a real decision on, because sessions over-count the curious and under-count the loyal.