Synonyms (in ancient Greek, συν ("syn") = plus and όνομα ("onoma") = name) 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.
An example of synonyms are the words car and automobile. 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:
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. However, many people feel that the synonyms they use are identical in meaning for all practical purposes, and are interchangeable. 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, such as a long arm and 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.

baby and infant (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) Related terms
Antonyms are words with opposite or nearly opposite meanings. For example:
The words synonym and antonym are themselves antonyms.
Hypernyms and hyponyms are words that refer to, respectively, a general category and a specific instance of that category. For example, vehicle is a hypernym of car, and car is a hyponym of vehicle.

dead and alive (compare to synonyms: dead and deceased)
near and far (compare to synonyms: near and close)
war and peace (compare to synonyms: war and armed conflict)