The Greek letter (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 .
The capital Greek letter Θ (capital theta).
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.
The tangent function returns the tangent ratio of the input angle. In geometric terms, the function returns the length of the line tangent to the point on the unit circle.