The Greek letter (theta) is used in mathematics 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 .
The cosine function returns the cosine of an angle provided in radians. In geometric terms, the function returns the ratio of the right-triangle's adjacent side over its hypotenuse.
The sine function returns the sine of a number provided in radians. In geometric terms, the function returns the vertical component of the point formed by the angle on the unit circle.
Given the angle of a right triangle as input returns the ratio of the triangle's opposite side over its adjacent side.
The capital Greek letter Θ (capital theta).