Experimental error is the difference between a measurement and the true value.
Experimental error has two components - systematic error and random error.
A systematic error (inaccuracy or bias) is a consistent, repeatable error in the same direction (error is always higher or always lower) due to faulty calibration of equipment or a flawed experiment design (eg. observer parallax error). It is also referred to as ‘determinate’ error as it is possible to determine and fix this type of error.
A random error (imprecision) is an unpredictable error, in either direction (randomly higher or lower) due to unknown changes or fluctuations during the study. It is also referred to as ‘indeterminate’ error as it is not possible to determine and control this type of error.
Instrumental error is a subset of systematic error, due either to inherent faults of the equipment (eg. inaccurate ruler where the fix may need to be replacement) and/or incorrect calibration (eg. voltmeter).