A thetan* is the person himself -- not his body or his name, the physical universe, his mind or anything else, that which is aware of being aware, the identity which IS the individual. The thetan is most familiar to one and all as you.

L. Ron Hubbard

* Thetan: The term thetan was coined to eliminate any possible confusion with older, invalid concepts. It comes from the Greek letter theta which the Greeks used to represent thought or perhaps spirit, to which an n is added to make a noun in ht modern style used to create words in engineering.