From AMS Glossary
The difference between the true value of some quantity and its observed value.
Every observation is subject to certain errors, as follows. 1) Systematic errors affect the whole of a series of observations in nearly the same way. For example, the scale of an instrument may be out of adjustment. These instrument errors can be detected and corrected by comparison with a standard. The personal equation of an observer may lead him to make small systematic errors in his readings; for example, if the scale is not at eye level. 2) Random errors, which appear in any series of observations, are generally small and as likely to be positive as negative; their magnitudes are usually distributed according to the error distribution. 3) Mistakes are widely discrepant readings.