The word hexadecimal refers to a number system with 16 as a base, unlike the metric system, which has a base of 10.
The metric system has a base of 10: the number 10 forms patterns for the entire system, whether you're measuring distance, weight, or volume. However, as basic as 10 may seem, it's not the only possible basis for a number system: A system based on 16 is a hexadecimal system. There aren't a lot of uses for this word elsewhere, but it is used by computer programmers, where it’s common enough to be abbreviated as hex.