The ability to show multiple ads in a single location, varying the treatment displayed on each new page load and/or within a single page load. Ads are generally rotated to either avoid consumer wear-out or as a part of ad optimisation and testing. 

Describes the rotation of advertisements on a web page - each time a user clicks on a different page or returns to a page they've viewed previously in the same session, a different [..]

A Google AdWords setting that determines how often Google delivers your active ads in relation to one another within an ad group, whether optimized either for clicks or conversions, or rotated more evenly (Source)

A preference that determines which ad in your ad group should show when you have multiple ads active. Rotation settings include Optimize for Clicks