A lowercase Greek letter theta commonly used to represent angles, phases, or parameters in geometry and physics.
Examples
Angular measurement in trigonometry
\sin(\thetasym) = \frac{\text{opposite}}{\text{hypotenuse}}
Phase angle in complex analysis
z = re^{i\thetasym} = r(\cos \thetasym + i\sin \thetasym)
Angle in polar coordinates
(r,\thetasym) = (\sqrt{x^2 + y^2}, \arctan(\frac{y}{x}))