Sticky bit.

If set on a file, this is currently meaningless, but in years past it requested that the operating system keep the program's code in memory, thus speeding up further program invocations. This is no longer necessary with virtual memory.

If set on a directory, a file in the directory can be removed or renamed only if the user has write permission for the directory, and either:

• owns the file
• owns the directory, or
• is the superuser (root).