A synonym (a word which comes from the Greek sun, 'with', and onoma, 'name') is a word with the same - or virtually the same - meaning as another word. The words may be substituted for each other depending on the context; for example, the verbs 'to live' and 'to dwell'. The choice of one synonym rather than another depends on the context, and often is a matter of connotation and register rather than denotation. It is at the heart of stylistic free choice.
Words that can be interchanged can be described by the related adjective synonymous. The use of the word synonymous is often figurative in English. For example, people can often be heard to say that a particular brand name (e.g., Coca-Cola) is "synonymous with quality", meaning (figuratively) that the brand's products represent all that can be desired, rather than using the word synonymous with its literal meaning: we cannot say that "The paper was of good
Coca-Cola" when we mean "The paper was of good quality".
The opposite of synonyms are antonyms - words that have the opposite or nearly the opposite meanings. An example of these: hot is the antonym of cold.
Note on Pronunciation: while the noun synonym is pronounced with the stress on the first syllable, IPA: /'sɪ nə nɪm/, the adjective synonymous has the stress on the second syllable, IPA: /sɪ 'nɒ nɪ məs/.
See also Words ending -onym, -onymous, -onymy.