Session in Google Analytics
 

A session is simply a collection of hits, from the same user, grouped together. By default, most analytics tools, including Google Analytics, will group hits together based on activity. When the analytics tool detects that the user is no longer active it will terminate the session and start a new one when the user becomes active.

Most analytics tools use 30 minutes of inactivity to separate sessions. This 30-minute period is called the timeout.