ΨΦ

 Psi ( 'Ψ') and Phi ('Φ') are the 23rd and 21st letters of the Greek alphabet.

The phonetic pronunciation of ΨΦ in American English is the same as "sci-fi": a corrupted abbreviation for "science fiction." As such, ΨΦ occasionally appears in science fiction as an in-joke.

In the novel Fallen Angels, ΨΦ is used to identify various characters as belonging to an underground resistance movement consisting of science fiction fans.