The difference between nominal dimensions and actual dimensions is that nominal dimensions are the dimensions that are used for the design and/or identification of an object, while actual dimensions are the dimensions that have been measured and/or fabricated. Nominal dimensions are typically specified in design drawings and blueprints, while actual dimensions can be determined through measurement or inspection.
Nominal dimensions are usually approximate, and are not necessarily the same as the actual dimensions of the object. This is often due to manufacturing tolerances, which allow for small variations in the size of the object. Actual dimensions, on the other hand, are the exact measurements of an object.
For example, a nominal dimension of a part of a machinery might be 8 inch, and the actual dimension of it measured in real-world scenario could be 8.2 inch.