Restriction on the amount of times a specific visitor is shown a particular advertisement.
Frequency capping is often cited as a way to avoid banner burnout, the point where visitors are being overexposed and response drops. This may be true for campaigns of a direct-response nature measured by click-throughs, but it might run counter to campaigns of a brand-building nature measured by non-click activity.
Frequency, Not Format
ClickZ (August 21, 2002)
Rethinking Low Frequency Caps
ClickZ (February 21, 2001)