A follower of L. Ron Hubbard and the "Church" of
Scientology.
The term is a play on "Cylons," the robotic invaders in Battlestar Galactica. It is particularly appropriate because Hubbard was a science fiction writer, and the top-seekrit "OT levels" are based on the belief that we are infected with "body thetans," the confused, immortal souls of
space aliens who became disoriented 75,000,000 years ago when the
evil space warlord Xenu brought them to Earth (or Teegeeack, as it was known then), dropped them into volcanoes and blew them up with nuclear weapons.