Frequency

The number of times the target audience is typically exposed to content, advertising, or a specific ad, in the defined time frame. Frequency represents the average exposure when used in conjunction with cumulative reach estimates, though it can also be reported on the basis of specific exposure levels when evaluated in the context of discrete reach estimates through frequency distribution analyses. (Source: MRC Digital Place-Based Audience Measurement Standards Version 1) In a similar vein, Effective Frequency refers to the number of exposures necessary to make an impact and attain communication goals.