What does A Sticky mean in SEO?

When a forum discussion (often called a "Thread") is felt, by the moderators, to be extremely useful they can mark it so that it is one of the first discussions to show up in a certain section of the forum.

While most discussions are shown in date order (with the discussion that has had the most recent comment added showing at the top) "stickies" over ride this and are always shown at the top.

To see this in action visit this page:
http://forums.seochat.com/google-optimization-7/

In this case the first few threads are actually marked as "Sticky" although many forums use a graphic to illustrate rather than literal text.

The term refers to the way they "stick" at the top even if they are very old.

Creating a thread that becomes a sticky can give your website a valuable backlink if your link is within the thread or even in the signature. This is because sticky threads are by their nature highly useful and so tend to gain backlinks of their own.