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.