28 Day Active Users is a Metric in Google Analytics under the User section
28 Day Active Users Definition: Total number of 28-day active users for each day in the requested time period. At least one of ga:nthDay, ga:date, or ga:day must be specified as a dimension to query this metric. For a given date, the returned value will be the total number of unique users for the 28-day period ending on the given date.
Data Analytics & Insights Google Analytics Metrics 28 Day Active Users