I agree with user17237 that you should write out one of the words, but I'd go the other way:
The supplied definition defines 24 sixteen-bit words ...
Or, if you don't mind a change of unit,
The supplied definition defines 24 two-byte words ...
(It would be nice if you could break up "definition defines" somehow too; perhaps something like lays out could replace defines.)
Edit: As another alternative to placate the naysayers, perhaps
... defines 24 (16-bit) words ...
would suffice, since the most common bitsize of a word is 16 bits, and you're just reminding them of it?