"Unique Audience" is the total number of visitors who visit a website or use an application at least once in a given period. Multiple visits to the website or application within the same time frame are only counted once.
Unique Audience helps provide a picture of the reach of a particular website, whereas 'visits' help provide a picture of frequency and engagement.
Also referred to as "Unique Visitors."
Source: Nielsen/Net Ratings, Hitwise