A denomination is a proper description of a currency amount. it is oftentimes a sub-unit of the currency. for example, traditionally a cent is 1/100th of a particular unit of currency.).
Monero denomination names add SI prefixes after dropping the initial "mo" for ease of use. The smallest unit of Monero is 1 piconero (0.000000000001 XMR).
Support for input using SI prefixes was added to the Monero codebase on March 3, 2017 by Moneromooo. The smallest unit of Monero (10^-12 XMR) was originally called a tacoshi in honor of user Tacotime, an early Monero contributor and was later renamed for ease of use and consistancy.