A pre-packaged exploit of a known security vulnerability that is distributed far and wide. Once someone packages a rootkit for a particular vulnerability, legions of clueless script kiddies will busily attempt to apply it to everything in sight.
It is important to patch security holes as soon as you know about them. It is ABSOLUTELY VITAL to patch them before a rootkit makes its way out to the script kiddies - no matter how obscure you think you are, it's an outright guarantee that you aren't so obscure that some random kiddie won't try that rootkit against your server or network at random.
See also chkrootkit.