Tolerance is an engineering term that refers to the allowable limit of variation in a measurable property of an object. For instance, to fit correctly and to perform its task well, an automotive engine component may need to be manufactured to a high level of dimensional precision. In that case, it will have a small size tolerance. Another component may need to operate within a wide variety of heat conditions, so it would have a large temperature range tolerance.
Whatever the quality being measured, a tolerance is usually specified in terms of a plus or minus percentage variation from a specified measurement (+/-%). For example, if a resistor with a value of 1000 ohms and a tolerance of plus or minus 5% is required, then any resistor with a resistance from 950-1050 ohms would be acceptable for that task.
It is generally more expensive and/or more difficult to manufacture items to finer degrees of tolerance, so it is important to know what range of tolerance is acceptable in any given circumstances, so that manufacturing costs can be minimized.
