Meaning of the word sy in English

What does sy mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

sy

US /saɪ/
UK /saɪ/

Noun

the 23rd letter of the Greek alphabet (Ψ, ψ)

Example:
The symbol for psychology is the Greek letter psi.
In physics, psi often represents a wave function.