# psi

/ˈsaɪ/ (US) · /sˈaɪ/ (UK)

Source: https://englishreference.com/word/psi
Updated: 2026-08-08

## Senses

### 1. unit of pressure

noun · CEFR B2 · Technical

Stands for pounds per square inch. You use this to measure how much air or liquid pressure is inside something, like a car tyre.

For teachers: Abbreviation for pounds per square inch. A unit of pressure used primarily in American and British engineering contexts to measure the force exerted on a surface.

- You should check that your car tyres are filled to 32 psi before a long trip.
- The industrial boiler is rated for a maximum operating pressure of 150 psi, beyond which the safety valves are designed to trigger automatically.

Usage: Usually written in lowercase as 'psi' when used as a unit of measurement.

### 2. Greek letter

noun · CEFR C1 · Technical

the twenty-third letter of the Greek alphabet (Ψ, ψ).

For teachers: The twenty-third letter of the Greek alphabet, often used in mathematics and physics to represent specific variables or functions.

- The professor wrote the Greek letter psi on the board to represent the wave function.
- In quantum mechanics, the Greek letter psi is the standard symbol used to denote the state of a system within a wave equation.

Usage: Capitalised as 'Psi' when referring to the letter name in a sentence.

### 3. psychic energy

noun · CEFR C2 · Technical

a word used to describe mysterious mental powers, like reading minds or moving objects with your brain.

For teachers: A general term for parapsychological phenomena, including extrasensory perception and psychokinesis.

- The movie was about a girl who could use psi to move objects across the room.
- Early researchers in parapsychology attempted to quantify psi through rigorous card-guessing experiments, though the results remained highly controversial in the scientific community.

Usage: Used as an uncountable noun to refer to the hypothetical force itself.

## Examples

- Check your car tires to see if they have enough PSI.
- The manufacturer recommends maintaining a tire pressure of 32 PSI for optimal fuel efficiency and safety.
- Industrial boilers are equipped with safety valves designed to release steam automatically if the internal pressure exceeds a specific PSI threshold, preventing catastrophic structural failure.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.