A capital Greek letter Theta commonly used to represent angles, transformations, or parameter sets in mathematics and physics.
Examples
Big-O notation complexity bound
T(n) = \Theta(n^2)
Angle in trigonometry
\Theta = \frac{\pi}{4} \text{ radians}
Parameter in statistical model
L(\Theta | X) = \prod_{i=1}^n f(x_i | \Theta)