An estimator is any quantity calculated from the sample data which is used to give information about an unknown quantity in the population. For example, the sample mean is an estimator of the population mean.
Estimators of population parameters are sometimes distinguished from the true value by using the symbol 'hat'. For example, true population standard deviation is estimated (from a sample) population standard deviation
Example: The usual estimator of the population mean is where n is the size of the sample and x1,x2, …,xn are the values of the sample. If the value of the estimator in a particular sample is found to be 5, then 5 is the estimate of the population mean µ.