Estimator

Rule for calculating an estimate of a given quantity based on observed data

In statistics, an estimator is a rule for calculating an estimate of a given quantity based on observed data: thus the rule (the estimator), the quantity of interest (the estimand) and its result (the estimate) are distinguished. For example, the sample mean is a commonly used estimator of the population mean. There are point and interval estimators.

From Wikipedia, under CC BY-SA. More on occurri.