An engineering term that refers to the specification of a range of dimension (size) that a part is required to meet. This range of dimension is referred to as the ideal or target size, plus or minus an acceptable variance. It is unrealistic, and inherently difficult, to plan to make multiple of the same part, or different mating parts, to one exact dimension. For example, if you make a 10mm hole, a 10mm shaft will not be able to fit in it. There needs to be some free space between the parts in order for them to fit together. This free space is another kind of tolerance. The higher the required tolerance for a part, the smaller the required range of size, and the more difficult it is to make. The lower the required tolerance, the larger the acceptable range of size, and the easier a part is to make.