An error has different meanings in different domains. The meaning in the some of those domains are described below.
An error is a difference between a computed, estimated, or measured value and the true, specified, or theoretically correct value. See also error function
An error is a difference between desired and actual performance. Engineers often seek to design systems in such a way as to mitigate or even avoid the effects of error. See also Observational error
An error is a deviation from a correct value caused by a malfunction in a system or a functional unit. An example would be the occurrence of a wrong bit caused by an equipment malfunction. (Sources: Federal Standard 1037C and MIL-STD-188). See also error-correcting code and error-detecting code
An individual language user's deviations from standard language paradigms are sometimes referred to as errors. At present, this usage is not widely used, as it connotes a value judgement about what what linguistic forms should and should not be used, something many modern linguists seek to avoid.
An error is said to occur when perfect fidelity is lost in the copying of information. For example, in an asexually reproducing species, an error has occurred for each DNA nucleotide that differs between the child and the parent. Errors in this sense are not judged as "good" or "bad", although an error may make an organism either more or less adapted to its environment.
An error is judged by the official scorer when A runner advances a base because of a fielding mistake; Perfect play would have prevented the advancement; and The mistake was physical. Mental misjudgements are not errors. Failing to get more than one out on given play is not an error. The enforcment of this rule is necessarily subjective.
See also: Mistake, trial and error, measurement error