Product metrics and goals
Active users (DAU, WAU, MAU)
Also called DAU, WAU, MAU, Daily active users, Monthly active users
The number of different people who used a product meaningfully in a day, week or month.
How it's worked out
Unique people who did a defined meaningful action in the period
The value of an active user count depends entirely on what 'active' means. Opening the app is weak; doing the thing the product is for is strong.
Pick the period that matches how often people need the product: daily for messaging, weekly for team tools, monthly for something like accounting.
In practice
A budgeting app counts someone as active in a week if they reviewed or categorised at least one transaction.
Common mistake
Counting logins or page views as activity. A product can grow its 'active users' while fewer people actually get value.