absolute error

Short definition from Wiktionary

Noun

1. 1
   The difference (sometimes defined to be the absolute difference) between the measured or inferred value of a quantity and its true value.
2. 2
   Shortest distance between the center of impact or the center of burst of a group of shots, and the point of impact or burst of a single shot within the group.
3. 3
   Error of a sight consisting of its error in relation to a master service sight with which it is tested, including the known error of the master service sight.