Definition of "thetan"

Actually a thetan is in a very, very small amount of mass. From some experiments conducted about fifteen or twenty years ago – a thetan weighed about 1.5 ounces!

1972, Report of the Commission of Enquiry into Scientology for 1972 (R. P. 55/1973), Pretoria: Government Printer, page 198

According to Scientology doctrine, each human being is actually a thetan or spirit, an immortal, godlike expression of the life force (theta). […] In his writings, Hubbard traces the existence of this life force back trillions of years. In the beginning theta was separate from the physical universe. Theta had no energy or mass, time or location; it was simply energy. Thetans therefore existed before and are the original source of the material universe.

2009, David G. Bromley, “Making Sense of Scientology: Prophetic, Contractual Religion”, in James R. Lewis, editor, Scientology, Oxford: Oxford University Press, page 91

One of the things Hubbard claimed he had "discovered" was that the essence of the human being was a spiritual being. This spiritual being, which is in itself good and fully conscious, Hubbard called a "thetan." From this deeper understanding of the human essence, where he saw survival as the fundamental urge for all thetans, he also discovered that the four dynamics of Dianetics had to be expanded to eight: […]

2014, Kjersti Hellesøy, “Scientology: The Making of a Religion”, in James R. Lewis, Jesper Aa[gaard] Petersen, editors, Controversial New Religions, 2nd edition, Oxford: Oxford University Press, page 260