Thetan

In Scientology, the concept of the thetan (/ˈθeɪtən/) is similar to the concept of self, or the spirit or soul as found in several other belief systems. The term is derived from the Greek letter Θ, theta, which in Scientology beliefs represents "the source of life, or life itself." In Scientology it is believed that it is the thetan, not the central nervous system, which commands the body through communication points.

Thetans have been described in the Church of Scientology in a number of ways.

According to the Church of Scientology, the concept for the thetan was first discovered in the early 1950s by L. Ron Hubbard, drawing on reports by Dianetics practitioners of past-life experiences. Although the term is comparable to a soul, a thetan can be connected to multiple people over time. An important goal in Scientology is to become one with the thetan as an Operating Thetan.

The term and concept were defined by Scientology founder L. Ron Hubbard, who adopted the Greek letter theta (Θ) to represent "the source of life and life itself". Hubbard first introduced his ideas of "theta-beings" in a lecture series of March 1952. He attributed the coining of the word to his wife Mary Sue. As an essential point of Scientology doctrine, a person's identity and self-awareness come entirely from a "thetan". It is redundant to refer to "a person's thetan," because the person does not exist independently.

...
Wikipedia