a monetary unit that is valued at a fraction (usually one hundredth) of the basic monetary unit
"The penny serves as a subunit in the British currency system, representing one-hundredth of a pound."
Any subdivision of a larger unit.
"The liver is composed of many smaller functional units called subunits that work together to filter blood."
"The mitochondria contains several protein subunits that work together to produce energy for the cell."
The word subunit comes from combining the prefix sub- with unit. It was formed to describe a part that is smaller than or subordinate to a whole unit.