(1) The average number of times households or persons viewed a given program, station or advertisement during a specific time period, such as one month. This number is derived by dividing the Gross Rating Points (GRP) by the total non-duplicated audience (cume). For example, if a group of programs achieved 30 GRP’s and a cume of 20, then the average frequency would be 1.5 exposures per person or household.
(2) A measure of Internet usage. Frequency can be expressed either in average minutes per user per month, or in average page views per month. Average minutes per user per month measures the average amount of time that the average visitor to the site spends on the site during the month. Average page views represents the average number of pages of pages on a Web site or properly viewed by the average visitor to the site during the month. (Source: eyada.com)
Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf