Dissemination of a Marketing message through just about any means, including shoving product samples down your throat.
There are basically two ways to do it on the Web these days:
- Obnoxious banner and Flash pop-ups that get in the way of readers
- Unobtrusive text-based, context-sensitive ads (like Google/AdSense)
The more aggressive trends in RSS advertising are triply obnoxious because advertisements are often inserted in a way that changes the feed item contents and/or checksums, forcing RSS aggregators to consider old posts as "new" or "changed" and making a mess of the reading experience - there are sensible ways to do it.