a unit of pressure
"The tire pressure was measured at 32 psi to ensure safe driving conditions."
the 23rd letter of the Greek alphabet
"The symbol psi represents the twenty-third letter of the Greek alphabet and is commonly used in mathematics to denote a wave function."
The twenty-third letter of Classical and Modern Greek and the twenty-fifth letter of Old and Ancient Greek.
"The judge reviewed the defendant's PSI before deciding on the prison term."
Initialism of pre-sentence investigation.
In plain English: Psi is an old-fashioned unit used to measure pressure, especially when talking about how hard gas pushes against something inside a container or tire.
"The psychic claimed she could read minds with her psi abilities, but no one believed a word she said."
Usage: Psi is the 23rd letter in modern Greek alphabets, positioned between omega and chi. It represents a voiceless labial-velar fricative sound that does not exist in English pronunciation.
Pounds per square inch (an imperial unit of pressure)
"The tire pressure was measured at 32 psi to ensure safe driving conditions."
The word comes to us via Middle English and Latin directly from ancient Greece. It originally referred specifically to the twenty-third letter of their alphabet before becoming known as a symbol in modern contexts.