Secondly and perhaps more importantly for the matter at hand, "we the experts" tend to hold more than one representation of language, often times without us being fully aware of our own internal range. In practice, one or more prototypical definitions of language coexist in our minds with other less central conceptions of language.
With these caveats in mind, I offer seven approximations to what I think language is or may be:
1) The number one (most popular) definition that we commonly confer to language is “means of communication,” which is heavily shaped by the conduit metaphor: “means,” “medium,” “channel,” “intermediary,” “avenue,” “route,” “path,” etc. of communication.
2) A secondary, less often discussed, yet comparably important definition that we hold in our minds is that of a “conceptualization system.” That is, a system to think about the world, a system that “structures and builds our reality”: how and what we think the world is.
3) A third conception of language is a “social construct.” In this definition, language is what it is only relative to its role in the formation and maintenance (sometimes destruction) of social bonds. Language is “whatever is shared by the members of a society”: common ground, mutual understanding, meaning negotiation, etc.
4) A fourth meaning associated with language is “set of linguistic symbols.” This places an emphasis on the symbolic nature of language. Different definitions of “symbolism”/“symbolization” come into play though. Some would go as far as to tell that human beings are “symbolic creatures,” which is not untrue, but such definitions lean to a rather ethereal shade of conversation. Others are more technical and define symbols as arbitrary units of mental representation: Fully detached “signs” for the representation of referential and non-referential objects of conception.
5) A fifth idea linked to language is “words.” This is probably the, ironically, least self-conscious definition of language. Hardly anybody would deny that language has words; yet, not many would be willing to give such a seemingly barbaric definition; however, in practice, our daily experience with language is an experience with words (which is what you are reading right now (funnily enough)). So, it is just fair that we have this unaware conception of “language as words.”
6) Quite inadvertently as well, our conception of language heavily depends on the ideas we indoctrinate ourselves with from our favorite authors. The more I read and the more I agree with a certain author, the more I will tend to define language in her or his own terms.
As can be seen thus far, language is liable to different conceptions and definitions. Different metaphors can be evoked to try to organize these different ideas into one coherent prototype cline. The one that seems most comfortable and practical (both conceptually and didactically) to me is the “radial metaphor,” in which we have a central core definition of language (the prototype); and then from there, other definitions extend outwards towards the periphery: