The Free On-line Dictionary of Computing (30 December 2018):

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)