A metric in Google Analytics that measures one user interacting with a website during a given period of time, A session may include several web page visits and interactions or it may contain just one. A new session begins if a user visited a website via one campaign, leaves and comes back via a different campaign. A session is not dependent on how many pages are viewed, so if a person goes to a website and looks around at different pages for 20 minutes, it would count as 1 session.
A new session will also start after midnight or after 30 minutes of inactivity. This means a single user might have multiple sessions in a single day or over several weeks.