The main sources of synonymy are: polysemy, word and phraseologisms formation and lexical borrowing. In the case of polysemy, one can make a connection between the synonym series of the most different systems, thus obtaining a chain that has links in all synonym types and in a large number of languages. For the most part, the synonym dictionaries consist of series of synonyms grouped according to the secondary meaning of the polysemantic words. A synonym and a polysemantic word which may have as many synonyms as secondary meanings they have correspond to each secondary meaning. As far as word and expression formation is concerned (having composition and metaphorization as processes), after they adapt, they enter in a certain relation with those existing in the borrowing language, including in synonymy relations. The lexico-semantic system of a language also enriches by means of borrowings from other languages: direct borrowings and borrowings from lexical distance matrices, borrowings in patois and borrowings in literary language or in technico-scientific terminology.