Tsukiyomi is a Shinto deity, brother or Amaterasu and Susanowo. He ruled over the moon & night in the Shinto religion. He appears only in two myths, one where he is born opposite his sister, Amaterasu, and with his brother Susanowo, and one where he kills a food goddess.
A special genjutsu (illusion) skill that Itachi is able to use with his Sharingan eyes. What it does it traps the victim into his mind where he is able to torture them mentally for what seems like days over a period of a few seconds.
Itachi uses this move on Kakashi by trapping him into his Tsukiyomi world and repeatedly stabbing him over a period of 72 hours which occurs in just a couple of seconds