This ad unit, often referred to as simply a “rectangle” or a “300×250,” is one of the most common display ad sizes. It is found on almost every website that monetizes through display ads and used by almost every advertiser running an online campaign.
The medium rectangle is a component of the IAB’s Universal Ad Package. Below is an example of a medium rectangle ad unit on HuffingtonPost.com:
There are a number of other rectangular ad units that are also somewhat common in display advertising campaigns:
Generally, horizontal ad units are referred to as “banners” to distinguish them from rectangle units (though banner units such as the 728×90 leaderboard are, of course, technically rectangles as well).