A rectangular graphic image which is included on a content page for advertising and promotional purposes.

In Interchange, when "banner" or "ad" is used to describe the functionality of the [banner] tag and it does not necessarily mean an image, as you can put anything in the content placeholder. In fact, banner examples from ???? HOW-TO use plain text.

Voluntary ad unit guidelines as suggested by the Interactive Advertising Bureau (specified in pixels, in ascending order):

You can see the above dimensions in practice at MOTIVE Resources.