A session is a specific block of time dedicated to something. Court may be in session, or school may be in session, or you just might find yourself enjoying a massage therapy session.
The noun session also means a meeting of a board or committee to conduct business. The school board, for example, may hold a special session to discuss budget issues. Session can also refer to a gathering of spiritualists at a séance or a meeting with a psychic. You might have a session with a medium who claims she's delivering a message to you from your great-great-grandmother.