An estimator
is a statistic
that represents the properties of a population.
Several estimators may be available to represent
a particular property.
When selecting the one you
prefer you would might consider the efficiency
and bias
of the alternatives.
The most efficient estimator
is the one that gives the lowest expected
variance
of the error. Technically the efficiency
is the lowest possible variance from any
estimator divided by the expected variance
of the selected estimator.
