Early inquiry into language can be traced back to as long ago as 1500 B.C. in India, long before any systematic description of language, and there were various schools of thought discussing linguistic issues in early medieval Indian philosophy (roughly between 5th to 10th Centuries A.D.)
In the Western tradition, the early work was covered, as usual, by Plato, Aristotle and the Stoics of Ancient Greece. Plato generally considered that the names of things are determined by nature, with each phoneme (the smallest structural unit that distinguishes meaning) representing basic ideas or sentiments, and that convention only has a small part to play. Aristotle held that the meaning of a predicate (the way a subject is modified or described in a sentence) is established through an abstraction of the similarities between various individual things (a theory later known as Nominalism). His assumption that these similarities are constituted by a real commonality of form, however, also makes him a proponent of Moderate Realism.
The Stoic philosophers made important contributions to the analysis of grammar, distinguishing five parts of speech: nouns, verbs, appellatives, conjunctions and articles. What they called the lektón (the meaning, or sense, of every term) gave rise to the important concept of the proposition of a sentence (its ability to be considered an assertion, which can be either true or false).
The Scholastics of the Medieval era were greatly interested in the subtleties of language and its usage, provoked to some extent by the necessity of translating Greek texts into Latin, with Peter Abelard, William of Ockham and John Duns Scotus meriting particular mention. They considered Logic to be a "science of language", and anticipated many of the most interesting problems of modern Philosophy of Language, including the phenomena of vagueness and ambiguity, the doctrines of proper and improper suppositio (the interpretation of a term in a specific context), and the study of categorematic and syncategorematic words and terms.
Linguists of the Renaissance period were particularly interested in the idea of a philosophical language (or universal language), spurred on by the gradual discovery in the West of Chinese characters and Egyptian hieroglyphs.
Language finally began to play a more central role in Western philosophy in the late 19th Century, and even more so in the 20th Century, especially after the publication of "Cours de linguistique générale" by Ferdinand de Saussure (1857 - 1913), which was published posthumously in 1916. For a time, in the 20th Century philosophical branches of Analytic Philosophy and Ordinary Language Philosophy circles, philosophy as a whole was understood to be purely a matter of Philosophy of Language.