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