Monthly Active Users (MAU) is a key performance indicator (KPI) for online games, mobile apps and social networking services.
MAU is calculated by taking the number of unique users for a thirty day period. It helps to determine the value of a company and finds the number of users who participate with and return to the site on a monthly basis.
Generally an active user is a person that is identifiable (by an ID, e-mail or username) for a company and is interacting with the corresponding business during a specified period. What activity specifically means in this context, needs to be defined individually and is largely depending on the offered service or product (e.g. a messaging service will define activity different to an e-mail provider).
It is important to notice that users who interact on a daily basis will as well appear in the weekly and monthly reports.
However, MAU is not suitable to measure the performance of all online businesses. Instead it takes different metrics to best reflect the size and usage of different networks and businesses.