(1)measurement error: measured quantity value minus a reference quantity value
NOTE 1 The concept of ‘measurement error’ can be used both
a) when there is a single reference quantity value to refer to, which occurs if a calibration is made by
means of a measurement standard with a measured quantity value having a negligible
measurement uncertainty or if a conventional quantity value is given, in which case the measurement error is known, and
b) if a measurand is supposed to be represented by a unique true quantity value or a set of true quantity values of negligible range, in which case the measurement error is not known.
NOTE 2 Measurement error should not be confused with production error or mistake.
(2) difference of quantity value obtained by measurement and true value of the measurand
(3) difference of forecast value and a, estimate of the true value
Note: (1) and (2) refer to measurement error, while (3) refers to a forecast error
(4) result of a measurement minus a true value of the measurand
NOTE 1 Since a true value cannot be determined, in practice a conventional true value is used
NOTE 2 When it is necessary to distinguish “error” from “relative error”, the former is sometimes called absolute error of measurement. This should not be confused with absolute value of error, which is the modulus of the error.
(2) CEOS/ISO:19159 [RD10]
(3) MACC [RD13]
(4) GUM [RD4]
Interestingly the GUM and VIM have slightly different definitions of "error". I find the GUM definition useful.