A session is a series of actions a single user who has visited, connected to, or logged into a website performs while browsing the website until he/she exits, disconnects from, or logs off the website. Sessions indicate the number of user sessions.
Its synonym is visits. Some use the term visit count, however the WACA does not recommend using it because it can be confused with the number of repeated visits. Be careful using some tools as their reports might confuse sessions with unique users.
If a single user views pages A and B over the course of a single session, the sessions and PVs are 1 and 2, respectively. A session generally terminates 30 minutes after the last access.