An accuracy is the degree to which a calculation, a measurement, or set of measurements agree with a true value or an accepted reference value. "accuracy includes a combination of random error (precision) and systematic error (bias) components which are due to sampling and analytical operations; a data quality indicator. EPA recommends that this term not be used and that precision and bias be used to convey the information usually associated with accuracy (QAMS 1993, 1)."

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further definition:
An Accuracy is the degree of agreement of a measurement (X) with an accepted reference or true value (T); usually expressed as the difference between the two values (X - T), or the difference as a percentage of the reference or true value (100[X - T]/T), and sometimes expressed as a ratio (X/T).

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further definition:
An accuracy is a measure of how close repeated trials are to the desired target.