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.
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.
"Those who work with language know that there is no such thing as a true synonym. Even though the meanings of two words may be the same - or nearly so - there are three characteristics of words that almost never coincide: frequency, distribution and connotation."
This is well reflected in various new English dictionaries where you find frequency data next to a dictionary entry, etc.
One of the major achievements in lexicography belongs to a Hungarian translator Tibor Bartos, who compiled his Magyar szótár by claiming the very same idea as above.
The words synonym and antonym are themselves antonyms.