The total number of households or individuals in a television audience viewing for two or more-time periods within a schedule of spots or programs without regard to duplication. For example, an individual is counted twice in gross audience if he/she appears in the audience of two of the spots or programs within the schedule.
Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf