Answer:

PSIS

Answer	Crossword Clue
PSIS	Greek characters
PSIS	Greek letters
PSIS	Omegas' preceders

PSIS	Trident-shaped symbols
PSIS	Pitchfork-shaped letters
PSIS	Fork-shaped frat letters
PSIS	Greek consonants
PSIS	Frat letters
PSIS	Sorority letters
PSIS	Trident-shaped letters
PSIS	Preceders of omegas

PSIS	Omega preceders
PSIS	Frat characters?
PSIS	Trident-shaped Greek letters
PSIS	Three-pronged letters
PSIS	Greek letters resembling pitchforks
PSIS	Chi chasers
PSIS	Pitchfork-shaped Greek letters
PSIS	Penultimate Greek letters

PSIS	Some Greek letters
PSIS	Letters before omegas
PSIS	Next-to-last Greek letters
PSIS	Letters that resemble pitchforks
PSIS	Athenian consonants resembling pitchforks
PSIS	Greek letters shaped like pitchforks
PSIS	Fraternity letters
PSIS	They look like pitchforks

PSIS	Fraternity characters
PSIS	Penultimate letters
PSIS	Quantum mechanics symbols
PSIS	They resemble pitchforks
PSIS	They resemble tridents
PSIS	Athenian consonants
PSIS	Particles of the meson family
PSIS	Greek letters.

PSIS	Letters from Athens
PSIS	Homeric letters
PSIS	Letters from Greece
PSIS	Purportedly psychic phenomena
PSIS	Electric flux symbols