Unbiased

A sample is said to be unbiased when
every individual has an equal chance of being chosen from the population.

An estimator is unbiased if it does not systematically overestimate or underestmate
the parameter it is estimating. In other words,
it is unbiased if the mean of the sampling
distribution of the statistic is the
paramter it is estimating, The sample mean is an unbiased estimate of the population
mean.

See: bias