In a very generic sense, language is a mode of communication based on symbolic reference (the way words refer to things) and involving combinatorial rules that comprise a system for representing synthetic logical relationships among these symbols. (from Terence Deacon The Symbolic Species) 

For Ferdinand de Saussure, "language is a form and not a substance." (Cours, p.169) (see form / matter ) 

For Terrence Deacon, all symbolic activities place an extraordinary cognitive computational demand on the brain. Correlational, or indexical associations, which can be based on immediate context and learned by rote, need to be unlearned in a particular way. (Charles Sanders Pierce had insisted that indexical signs act on the nervous system. "Anything which focusses the attention is an index. Anything that startles us in an index, in so far as it marks the junction between two portions of experience." -- Philosophical Writings, pp 108 - 109.) The highly developed prefontal cortex of the human brain allows humans to maintain something in short-term memory while simultaneously recoding it into a symbolic system of association. Deacon believes that symbol use itself has been the prime mover for the prefrontalization of the brain in hominid evolution

Language has been used to distinguish mankind from all other species, as the basis for the distinction between nature and culture. In general, in the Ancien Régime, the boundaries of man are much more uncertain and fluctuating than they will appear in the nineteenth century, after the development of the human sciences. Up until the eighteenth century, language jumps accross orders and classes, for it is suspected that even birds can talk. John Locke refers to the the Prince of Nassau's parrot -- which was able to hold a conversation and respond to questions "like a reasonable creature." (see Agamben, The Open, p. 24) The boundaries of the human were still threatened not only by real animals, but also by creatures of mythology: sirens, satyrs, sphinges, etc. 

Is language an evolved evolutionary adaptation? If so, it would link humans to other species as well as differentiating them. For Ernst Cassirer, the freedom of the human spirit is not a a freedom from the determinations of the animal world, but a recognition of those determinations.

According to Giambattista Vico, the ancient language, before the formation of society, must have been full of the boldest metaphor, since this is the natural character of "words taken wholly from rough Nature, and invented under some Passion, as Terror, Rage, or Want." A distant echo of Vico's theories can be heard in Steven Pinker's Darwinian accounts of language. For Pinker, metaphors of space and force are quite possibly part of our evolutionary inheritance and are so basic to language that they are hardly metaphors at all, at lease not in the literary sense. 

Is there a "mentalese," or language of thought? Gottfried Leibniz believed in the existence of a "remarkable thought", "that a kind of alphabet of human thoughts can be worked out and that everything can be discovered and judged by comparison of the letters of this alphabet and an analysis of the words made from them." Many contemporary cognitive scientists have concluded from their research that a handful of concepts about places, paths, motions, agency, and causation underlie the literal or figurative meanings of tens of thousands of words and constructions, not only in English, but in every other language that has been studied, (Stephen Pinker, How the Mind Works, p.335) 

Claude Levi-Strauss extended and transposed the structuralist conceptions of language to cultures. For Levi-Strauss, "Any culture may be looked upon as an ensemble of symbolic systems, in the front rank of which are to be found language, marriage laws, economic relations, art, science, and religion." (Introduction to Marcel Mauss). 

The great sacral cultures, such as Christendom, the Islamic Ummah, even the Middle Kingdom, were imaginable largely through the medium of a sacred language and written script. (Benedict Anderson.) Each considered its language not as an arbitrary sign system, but as the emanation of reality. For Michel Foucault, the impersonal "great murmur" of language, its language-being, varies in each historical formation.

Since J.L. Austin drew attention to the performative functions of language in How to Do Things with Words (1962), philosophers have become concerned with the effectiveness of language, not just its representational function. 

Jacques Lacan describes schizophrenia as a breakdown in the signifying chain of language, in the interlocking syntagmatic series of signifiers which constitute an utterance