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.