A variant form of the uppercase Greek letter Theta commonly used as a growth rate bound in complexity theory.
Examples
Big-O notation with Theta complexity
\text{The algorithm runs in } \varTheta(n \log n) \text{ time}
Angle in spherical coordinates
r(\varTheta, \phi) = \sqrt{x^2 + y^2 + z^2}
Angular displacement in physics
\varTheta(t) = \varTheta_0 + \omega t + \frac{1}{2}\alpha t^2