The Jargon File (version 4.4.7, 29 Dec 2003):

rootkit
 /root'kit/, n.

    [very common] A kit for maintaining root; an automated cracking tool.
    What script kiddies use. After a cracker has first broken in and gained
    root access, he or she will install modified binaries such as a modified
    version login with a backdoor, or a version of ps that will not report the
    cracker's processes). This is a rootkit.