Note: This definition is no longer necessary for Realm Cloud as it is now measured by simultaneous connections.  See here.  

A monthly active user (MAU) is defined as a unique user who authenticates against the Realm Cloud or the Realm Platform during a 30 day period.  While an application may have a user base of 100,000 users, MAUs would only encompass the subset of users who have accessed the application within the last 30 days.