The Greek letter θ (theta) is used in math as a variable to represent a measured angle. For example, the symbol theta appears in the three main trigonometric functions: sine, cosine, and tangent as the input variable. In plain language, this represents the cosine function which takes in one argument represented by the variable θ.

Theta (UK: / ˈθiːtə / ⓘ, US: / ˈθeɪtə /) uppercase Θ or ϴ; lowercase θ[note 1] or ϑ; Ancient Greek: θῆτα thē̂ta [tʰɛ̂ːta]; Modern: θήτα thī́ta [ˈθita]) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth 𐤈. In the system of Greek numerals, it has a value of 9.

In mathematics, the symbol θ (theta) is commonly used to represent an angle. This Greek letter is frequently used in trigonometry and other branches of mathematics to denote an unknown angle or a specific angle within a given problem.

Greek alphabet letters and symbols. Greek letters pronunciation.

How to work out Feta in a triangle by Samantha Ward - July 17, 2013

Commonly used alongside Theta to represent angles or coefficients in mathematical equations. Frequently used in mathematical and scientific equations. Often used in physics to represent the wavelength of a wave. Represents the mathematical constant Pi. Used to denote the sum of a series of terms in mathematics.

Theta (uppercase Θ / lowercase θ), is a letter in the Greek alphabet. It represents the "th" sound (as in "think") in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 9. The Cyrillic letter Ѳ came from theta.