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*.)