Definition of Imperial System
The imperial system is a traditional measurement system that uses units such as ounces, pints, quarts, and gallons. In cooking, it is most often discussed when comparing older British-style measurements with other systems.
It is important not to assume imperial and US customary units are always the same. Some unit names match, but the actual sizes can differ.
In practical kitchen use, the imperial system matters most when a recipe source or chart specifically refers to imperial measurements rather than US customary ones.