Base 16. Coined in the early 1950s 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” comes from the combining root of decem, Latin for 10. If wish to create a truly analogous word for base 16, we should start with sedecim, Latin for 16. Ergo, sedecimal is the word that would have been created by a Latin scholar. 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.