A denomination is a way of classifying things — it names the type or value of something. Denomination often refers to money. For example, $20 bills are of the same denomination.
In mathematics, it refers to unit - a grade or unit in a series of designations of value, weight, measure, etc.
"Unit" is a general term that means the type of measurement. Units can be grouped together to make a "System".
Going back to its definition, " a way of classifying things", the meter, kilogram and second (together with a few other units) together make up the "SI" Metric System of Measurement.
The inch, foot, yard and mile are the units of length in the US Standard System of Measurement.
See a glossary of math definitions for common and important mathematics terms used in arithmetic, geometry, and statistics.