the 8th letter of the Greek alphabet
"The symbol theta, which is the eighth letter of the Greek alphabet, often appears in mathematical formulas."
The eighth letter of the Modern Greek alphabet, ninth in Old Greek: Θ, θ.
"As a new theta, she felt honored to join her sisters at the annual dance."
A member of the Kappa Alpha Theta sorority.
In plain English: Theta is the fifth letter of the Greek alphabet, often used as a symbol for angles and variables in math and science.
"The teacher explained that theta is a Greek letter often used in math and science."
Usage: Use theta to refer specifically to the eighth letter of the modern Greek alphabet or its mathematical symbol representing an angle. Do not confuse this proper noun with generic terms for angles unless you are explicitly discussing Greek notation.
The word theta was borrowed from Ancient Greek, where it originally referred to the letter itself.