Tolerance is the deviation from a nominal value. It is the range in which the nominal value can move. Tolerances can be specified for physical quantities and electronic components, for circuits, measuring instruments and systems. Tolerance is a dimensionless number given as a part of the whole or as a percentage.
For example, if a resistor of 100 kilo-ohms has a tolerance of 2%, then the actual value of the resistor may be between 92 kilo-ohms and 102 kilo-ohms. The value tolerance of passive components is reflected in the standard values of the E series.