From Chaplain Care, on Scientology:

Scientologists follow Mr. Hubbard's belief that a person is neither mind nor body, but a spiritual being - a soul. However, the word "soul" is an ambiguous term, which had been given many meanings by many religions. In order to avoid confusion, Mr. Hubbard selected the word thetan from the Greek letter "theta" which has traditionally meant "thought" or "life". A Thetan is the essence of a person, One does not have a Thetan, one is a Thetan. The brain, and the rest of the body, is looked upon as a mechanism, a communication center for the Thetan. The mind is perceived as a collection of pictures.