Synonyms are usually defined as words similar in meaning, as words, that express the same idea. A group of synonyms is called a synonymic set. Each synonymic set has a word which expresses the most general idea and holds a commanding position over other words. It is called the synonymic dominant.
In traditional linguistics synonyms are defined on the basis of the notional criterion. According to it synonyms are «words of the same category of parts of speech conveying the same notion, but differing either in shades of meaning or in stylistic characteristics» (V.V. Vinogradov).
The definition of synonyms based on the semantic criterion runs as follows. Lexical synonyms are different words of the same part of speech (having the same grammatical distribution) which have some common denotational components) in their semantic structure, but differ either in some denotational component(s) or in some connotational component(s) and thus usually have different lexical collocability.
In modern research on synonyms the criterion of interchangeability is sometimes applied. According to it, synonyms are defined as words which are interchangeable at least in some contexts without any considerable alteration in denotational meaning. The application of this criterion is limited.
Synonyms have three main functions in speech: the function of substitution; the function of precision in meaning; the expressive, stylistic function.
Synonyms are traditionally divided into ideographic, stylistic and absolute.
