The degree of conformity of a measured or calculated value to the true value, typically based on a global reference system.
Accuracy and precision are two measures of observational error. Accuracy is how close a given set of measurements (observations or readings) are to their true value, while precision is how close the measurements are to each other.
Accuracy is a description of only systematic errors, a measure of statistical bias of a given measure of central tendency; low accuracy causes a difference between a result and a true value; ISO calls this trueness. Alternatively, accuracy means the combination of both types of observational error (random and systematic), so high accuracy requires both high precision and high trueness.