Source : Free On-Line Dictionary of Computing

sticky bit
     
         The {bit} in the mode of a {Unix} file
        which, if set for an executable, tells the {kernel} to keep
        the code loaded in {swap space} even after it has finished
        executing on the assumption that it is likely to be used again
        soon.  This performance optimisation was included in some
        early (and recent?) versions of {Unix} to save reloading
        frequently used programs such as the {shell} or {vi} from
        disk.
     
        If the sticky bit is set on a directory, an unprivileged user
        may not delete or rename files of other users in that
        directory even if he has write access to the directory.
     
        The Unix "ls" command displays a set sticky bit as a "t" in
        the permissions of a file or directory.
     
        (1997-02-26)