Synonyms are different words with identical or at least similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek ("syn") "with" and ("onoma") "name". The words car and automobile are synonyms. Similarly, if we talk about a long time or an extended time, long and extended become synonyms. In the figurative sense, two words are often said to be synonymous if they have the same connotation:

"a widespread impression that … Hollywood was synonymous with immorality" (Doris Kearns Goodwin)

Synonyms can be any part of speech (e.g. nouns, verbs, adjectives, adverbs or prepositions), as long as both members of the pair are the same part of speech. More examples of English synonyms are:

• baby and infant (noun)
• petty crime and misdemeanor (noun)
• student and pupil (noun)
• buy and purchase (verb)
• pretty and attractive (adjective)
• sick and ill (adjective)
• quickly and speedily (adverb)
• on and upon (preposition)
• freedom and liberty (noun)
• dead and deceased (adjective)

Note that the synonyms are defined with respect to certain senses of words; for instance, pupil as the "aperture in the iris of the eye" is not synonymous with student. Similarly, expired as "having lost validity" (as in grocery goods) doesn't necessarily mean death.

In English many synonyms evolved from a mixture of Norman French and English words, often with some words associated with the Saxon countryside ("folk", "freedom") and synonyms with the Norman nobility ("people", "liberty").

Some lexicographers claim that no synonyms have exactly the same meaning (in all contexts or social levels of language) because etymology, orthography, phonic qualities, ambiguous meanings, usage, etc. make them unique. Different words that are similar in meaning usually differ for a reason: feline is more formal than cat; long and extended are only synonyms in one usage and not in others (for example, a long arm is not the same as an extended arm). Synonyms are also a source of euphemisms.

The purpose of a thesaurus is to offer the user a listing of similar or related words; these are often, but not always, synonyms.

"...there is no such thing as a true synonym."

The use of a human natural language is a matter of agreement between people and names of things (words) are arbitrarily given to objects. Such names are meant to identify things, etc. and are therefore unique identifiers at the start, though may be longer than a single word. Hence you need disambiguation in defining the meaning of Wikipedia entry words too. So what you have is a list of words that may replace each other subject to various contextual circumstances.