synonym

A word which is similar enough in meaning to another word for it to be usable as a substitute in some contexts, such as illuminate and light. An antonym is a word which has the opposite meaning to another, such as light and dark. A hyponym is a word whose meaning is included within that of another, such as horse and animal. The study of sense relations of this kind is part of the subject of semantics.

(Synonym and antonym are antonyms.)

An example of synonyms are the words cat and feline. 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 (noun) student and pupil (noun) pretty and attractive (adjective) sick and ill (adjective) interesting and fascinating (adjective) quickly and speedily (adverb)

Note that the synonyms are defined with respect to certain senses of words;

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. long and extended are only synonyms in one usage and not in others, such as a long arm and an extended arm.

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)

In biology, synonym is used with a closely defined meaning, different for animals and plants, see synonym (zoology) and synonym (botany).