Larry Niven, Jerry Pournelle and Michael Flynn 1991 Fallen Angels

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.