retention:
Retention can be defined in 5 ways: 1. Full: which proportion come back every single day to the app until D+N. 2. Classic: which proportion come back to the app on Day+N 3. Rolling: which proportion come back to the app on Day+N or any day after that. 4. Return: which proportion come back to the app at least once within N days. 5. Bracket-dependent: a specific and restrictive case of return retention. http://www.applift.com/blog/user-retention.html