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.
$cos(θ)$
In plain language, this represents the cosine function which takes in one argument represented by the variable $θ$.
Symbol  Format  Data 

θ  Unicode 

$θ$  TeX 

SVG 