A session is a group of user interactions with your website that take place within a given time frame. A single user can open multiple sessions. Those sessions can occur on the same day, or over several days, weeks, or months. As soon as one session ends, there is then an opportunity to start a new session.
A session is equal to any portion of 15 minutes. For example: if a user stays on your website for 35 mins it will be calculated as 3 sessions.