In statistics a statistic is a number characteristic of some measure of a population, calculated from measurements of a sample of that population. A parameter is a number representing some characteristic of a population.

To take a very simple example, the mean age of the population of the United Kingdom is a parameter. It is long and tedious to calculate this number, so often in fact a sample is taken and the mean age of this sample is an estimate of the mean of the population. This estimate of the mean is a statistic.