An internet server dedicated to supplying advertising images for Web pages that include the appropriate HTML code.

This is how banner ads work (including the one at the top of your E2 page!) - a piece of code in the page's HTML tells your browser to go and fetch an image from a different server (the adserver), which supplies the page with an image picked at random (or, in the case of the more insidious DoubleClick adserver, analyses your browsing habits and chooses one that it thinks most likely to make you part with your money)

An example piece of adserver-calling HTML is the following, which E2 uses at the top of every page, to send you ads from an adserver called ''..

<!-- hey kids!  Time to pay the bills -->
<ILAYER height=80> </ILAYER>
src=",everything2" scrolling="no"     
marginwidth="0" marginheight="0" framespacing="0" frameborder="0"
width="480" height="80">
<a href=""><img
alt="Advertisement" width="468" height="60" border="0" 
align="top" hspace="10" vspace="10"></a>

Gotta love that comment at the top!
Although of course we love E2, it is technically possible to go get a program called AdFilter which invisibly removes banner ads from web pages - thereby making them load faster too!