An accent symbol placing a caret over theta, commonly used to denote parameter estimates in statistics and probability theory.
Examples
Maximum likelihood estimator notation
\hat{\theta} = \frac{1}{n}\sum_{i=1}^n x_i
Estimated angle in trigonometry or physics
\hat{\theta} = 30° \pm 2°
Statistical parameter estimation
P(|\hat{\theta} - \theta| < \epsilon) \geq 0.95