Synonyms are different words with similar or identical meanings and are interchangeable. Antonyms are words with opposite or nearly opposite meanings. (Synonym and antonym are antonyms.) An example of synonyms is 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 (noun)
• student and pupil (noun)
• pretty and attractive (adjective)
• sick and ill (adjective)
• interesting and fascinating (adjective)
• quickly and speedily (adverb).