A banner ad is, strictly speaking, a long rectangular image displayed on a webpage or other type of content (mobile application, digital game, etc) for advertising purposes. The typical banner ad is clikable and redirects users to the advertiser’s website.
The historical ad format is the full banner which is 468 pixels wide by 60 pixels high (468x60). The first ever banner ad was an ad for ATT and was displayed in 1994 on Hotwired website.
Other banner ad formats are the half banner (234X60 pixels) and the leaderbord (728X90).
In a broader sense, banner ad refers to all other graphic ad units (rectangles, buttons, skycraper,..).
Banner ads are dynamically delivered by ad servers and are sold through CPMs or CPCs.
See also banner blindness.