Psi

Psi, written Ψ (uppercase) or ψ (lowercase), is a letter of the Greek alphabet. The term is used as a generic label in many fields, where its meaning depends on context. In English-language texts it commonly designates specific concepts in science, mathematics, and everyday measurement.

In physics and mathematics, psi is often used to denote the quantum mechanical wave function, a complex-valued function whose squared magnitude gives a probability distribution for a system’s properties. The wave function depends on variables such as position and time and evolves according to equations like the Schrödinger equation. Psi is also used in other areas, for example as a symbol for the stream function in fluid dynamics or as a general state variable.

In parapsychology, psi is a shorthand term for purported psychic phenomena, including telepathy, clairvoyance, and psychokinesis. The field uses the label psi to group these phenomena, but such claims remain controversial. There is no consensus in the broader scientific community on robust, reproducible evidence for psi effects under standard scientific criteria.

In metrology, psi denotes the unit of pressure pounds per square inch. It is widely used in engineering and everyday life to describe tire pressures, gas pressures, and hydraulic systems. One psi equals approximately 6,894.76 pascals, or about 0.06895 bar.