A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words begin, start, commence, and initiate are all synonyms of one another because they can be used interchangeably in a sentence without changing its meaning. Synonyms can be any part of speech, including nouns, verbs, adjectives, adverbs, and prepositions. They are used to replace or clarify the meaning of a word, and they can help to make writing more precise and effective.

It is important to note that words are considered synonymous in only one particular sense. For example, long and extended in the context long time or extended time are synonymous, but long cannot be used in the phrase extended family. Synonyms with exactly the same meaning share a seme or denotational sememe, whereas those with inexactly similar meanings share a broader denotational or connotational sememe and thus overlap within a semantic field. The former are sometimes called cognitive synonyms and the latter, near-synonyms, plesionyms, or poecilonyms.

Antonyms, on the other hand, are words with opposite meanings. For example, an antonym of good is bad, while a synonym of good is fine. Understanding synonyms and antonyms is helpful when using a dictionary or thesaurus.