What is $\theta $

Hint: To explain this type of general questions we have to use logic like what does mean by x, y, z in mathematics.

Complete step-by-step answer:
Lets some point about $\theta $
1. The Greek letter theta is just used as a variable name, much like x and y.
    For some reason it is most commonly used to denote an angle.
    But no, it doesn't have a constant value.
2. θ is often used to represent angles in two different planes in three-dimensional geometry.
3. The most common use is as the angle coordinate in polar coordinates.

Additional information:
The Greek letter theta θ is used in mathematics as a variable usually associated with a measured angle. For example, the symbol theta appears in the three main trigonometric functions: sine, cosine, and tangent as the input variable

Note: Theta or $\theta $ it has a great role in all streams either science non science. Sometimes it has different meanings also according to their writing style.