A session is started when a unique user has opened the app and initialized Leanplum (calling Leanplum.start). The session end is defined by 30 minutes or more inactivity after the app is closed or backgrounded.
For example: If the user closes the app and returns 10 minutes later, it will roll into one session. If the user closes the app and returns 35 minutes later, it will count as 2 sessions. If the app remains open but inactive for 2 or more hours, the session will close.
Sessions are the basis of how many normalized metrics (e.g. purchases per day, events per user, etc) are calculated. Contact firstname.lastname@example.org with any additional questions on how sessions are defined.