The average number of times households or persons viewed a given program, station or advertisement during a specific time period. This number is derived by dividing the Gross Rating Points (GRPs) by the total non-duplicated audience (cume). For example, if a group of programs has achieved 30 GRPs and a cume of 20, then the average frequency is 1.5 exposures per person or household. The term is interchangeable with frequency.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf