Accuracy: how close results are to the true value
Precision: the repeatability of measurements

The accuracy and precision of measurements have special meanings in the fields of science, engineering, industry and statistics.

• The accuracy of a measurement system is how close it gets to a quantity's actual (true) value.

• The precision of a measurement system is the degree to which repeated measurements give the same results.

A measurement system can be accurate but not precise, precise but not accurate, neither, or both. For example, if an experiment contains a error in the way it is done, then increasing the sample size generally increases precision but does not improve accuracy. The end result would be a consistent, yet inaccurate, set of results from the flawed experiment. Eliminating the systematic error improves accuracy but does not change precision.