A piece of HTML on a webpage that will contact an ad server and ask for an ad.

The tag informs the browser to open a small window (say, 468×60 pixels), and in that window place whatever content is returned from some location like "http://ad.adserver.com/imp?Z=468×60&s".

Here is an example JavaScript ad tag.

<SCRIPT language='JavaScript1.1' src="http://ad.adserver.net/adj

/N4617.somdomain.com/B2850779;abr=!ie;sz=250x250;ord=[timestamp]?" >

</SCRIPT>