In digital advertising context, ad frequency capping is the process of limiting the number of times an individual is exposed to a single ad or campaign.
Ad frequency capping is used for preserving ROI for CPM campaigns - see banner fatigue - but it is also recommended for preventing users irritation which can be against brand building objectives.
Ad frequency capping may be especially an issue for some CPC campaigns, like retargeting campaigns, when some ad networks foster for ad impressions as their are only paid for clicks.
Capping is also a must for very intrusive ad units like home page takeovers. In this case capping is usually set for a single impression per visitor.
For CPM campaigns, ad frequency capping is often an option charged by ad networks and publishers.
Campaign capping is set at ad server level and is based on cookie usage. The maximum number of ad impressions for an individual can be specified for a period of time or for a session.
Frequency capping has some technical limits:
classic limits of cookies
it is complex to implement capping when different ad networks / publishers are involved
capping is partially ineffective for multi-device browsing
Ad frequency capping may be also used for sequential advertising.
See also session capping.