\hat{\theta}The symbol \hat{\theta} is used to denote an estimated value of a parameter, often in the context of statistics or signal processing. The 'hat' accent is used to indicate that the parameter is an estimator or an estimated value rather than the true value.
Sample mean as an estimator for the population mean
\hat{\theta} = \frac{1}{n} \sum_{i=1}^{n} X_iMaximum likelihood estimator for parameter \theta
\hat{\theta} = \arg\max_{\theta} L(\theta)Estimator for the slope in simple linear regression
\hat{\theta} = \theta_0 + \frac{\sum (X_i - \bar{X})(Y_i - \bar{Y})}{\sum (X_i - \bar{X})^2}