- The twenty-third letter of the Classical Greek and Modern Greek, the twenty-forth letter of Old Greek and Ancient Greek Alphabet.
- In mathematics (Ѱ, ѱ) is the angle between the tangent and the x-axis in the intrinsic coordinates system.
- The lower-case letter ψ is used as the symbol for the wave function in quantum mechanics.
- In high-energy physics, Ψ refers to a meson also called the J/Ψ particle. It is an unstable neutral/uncharged fundamental particle of the meson group that has a mass about 6,000 times the mass of an electron. See Wikipedia:List of subatomic particles#Mesons.
- Psi is also a popular Jabber client.
- Psi was also a character on the popular game and television series, Pokémon.
- The Italian Socialist Party (Partito Socialista Italiano)
- Pounds per square inch (PSI).
- It has come to be a symbol for (para)-psychology-related topics, and has also been used as a shorthand for supposed psychic or psionic abilities or powers.