ROT13 ("rotate by 13 places") is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the alphabet. It is a special case of the Caesar cipher, developed in ancient Rome.

Because there are 26 letters in the basic Latin alphabet, ROT13 is its own inverse; that is, to undo ROT13, the same algorithm is applied. It only affects alphabetic characters; numbers, symbols, and whitespace are left unchanged.

It is often used in online forums as a means of hiding spoilers, punchlines, puzzle solutions, and offensive materials from the casual glance.