Most sizes stated in catalogs and reference charts are 'Nominal' sizes. Nominal size is different than actual size.
Just as a "Two by Four" does not measure 2 inches by 4 inches, a 1/2 inch bolt does not measure 1/2 inch. Nominal size helps identify items and makes your job sometimes easier.
For example, you drill a hole using a 1/2 inch drill bit and then place a 1/2 inch bolt into the hole. You can only do this because your 1/2 inch bolt is smaller than 1/2 inch. If it were 1/2 inch it would never fit into the hole.
Identifying fasteners and parts by taking measurements can be confusing if you expect your measurements to be the same as the nominal diameter. A 1/2 inch bolt might measure .4930 inch.
In the US, almost all sizes expressed in inches and fractions of an inch are nominal. Whenever you see a size expressed in decimal (.4930), it is most likely actual size.
In engineering speak, Nominal size is a dimensionless designator
