Here, one may view the word 'meaning' to sort of mean a "language sufficient to express all ideas." In this sense, a "meaning" may be defined in terms of such an abstract language. Thus, a "function into the set of meanings" really means a function into the set of valid expressions in this hypothetical language.
I refer to a partial function because there may be certain sequences of ____ which are invalid. The domain of definition of this partial function may thus be considered to be the set of expressions which are "grammatically correct."
This question.... has to have an answer. The one characteristic common to all languages is that there exists a notion of sequence. Information has to be presented in a totally ordered fashion. Perhaps one day this will not be the case, but for now, we may assume this to be true. Thus, I don't believe this question has made any arbitrary choices. Being so fundamental, it must have an answer.
So... what word goes in the blank?