/hek-sə-ˈdes-məl/
n. Base 16. Coined in the early 1960s to replace earlier sexadecimal, which was too racy and amusing for stuffy IBM, and later adopted by the rest of the industry.
Actually, neither term is etymologically pure. If we take binary to be paradigmatic, the most etymologically correct term for base 10, for example, is denary, which comes from deni (ten at a time, ten each), a Latin distributive number; the corresponding term for base-16 would be something like sendenary. Decimal is from an ordinal number; the corresponding prefix for 6 would imply something like sextidecimal. The sexa- prefix is Latin but incorrect in this context, and hexa- is Greek. The word octal is similarly incorrect; a correct form would be octaval (to go with decimal), or octonary (to go with binary). If anyone ever implements a base-3 computer, computer scientists will be faced with the unprecedented dilemma of a choice between two *correct* forms; both ternary and trinary have a claim to this throne.