Synonyms (in ancient Greek syn 'συν' = plus and onoma 'όνομα' = name) are different words with similar or identical meanings. Antonyms are words with opposite or nearly opposite meanings. (Synonym and antonym are antonyms.)

An example of synonyms are the words cat and feline. Each describes any member of the family Felidae. 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 nouns, adverbs or adjectives, as long as both members of the pair are the same part of speech.

More examples of English synonyms:

baby and infant
student and pupil
pretty and attractive
smart and intelligent
sick and ill
funny and humorous
died and expired

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. Different words 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. In a way, hyponyms are similar to synonyms.

In contrast, antonyms (an opposite pair) would be:

• 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)
• tremendous and awful (compare to synonyms: tremendous and remarkable)

Homonyms are words that sound alike and can even be spelled the same but mean different things, such as to, too and two, or they're, there and their, or, fluke, fluke, and fluke. See article.