Tolerance in engineering is an allowance, given as a permissible range, in the nominal dimension or value specification of a manufactured object. The purpose of a tolerance is to provide leeway for imperfections or other uncontrollable factors such as temperature or line voltage. For example, an electrical specification might call for a resistor with a nominal value of 100Ω (ohms), but will also state a tolerance such as "±1%". This means that any resistor with a value in the range 99Ω to 101Ω is acceptable. It would not be reasonable to specify a resistor with a value of exactly 100Ω, because the resistance will vary with temperature and other factors in any case.
The tolerance may be specified as a factor or percentage of the nominal value, a maximum deviation from a nominal value, an explicit range of allowed values, or refer to a published standard where this information is specified. Tolerance can be symmetrical, as in 40±0.1, or asymmetrical, such as 40+0.2/-0.1.
It is good engineering practice to specify the largest possible tolerance while maintaining functionality, since closer or tighter tolerances are more difficult to manufacture and hence cost more to either build or buy.
Tolerance is related to, but different from fit, which is a designed in clearance or interference between two parts. For example, if a shaft with a nominal diameter of 10 millimeters is to have a sliding fit within a hole, the shaft might be specified with a tolerance range from 9.964 to 10 millimeters and the hole might be specified with a tolerance range from 10.04 to 10.076 millimeters. This would provide a clearance fit of somewhere between 0.04 millimeters (largest shaft paired with the smallest hole) and 0.112 millimeters (smallest shaft paired with the largest hole). In this case the size of the tolerance range for both the shaft and hole is chosen to be the same (0.036 millimeters), but this need not be the case in general.
Tolerance is different from safety factor, but an adequate safety factor will take into account relevant tolerances as well as other possible variations.