A synonym is word, with the same meaning as another word. For example, seldom and rarely are synonyms both words means “almost never”. Vocabulary tests often include multiple-choice question about synonyms. One type of multiple-choice question is a sentence with an underlined word or phrase. You read the sentence and then choose which of four words is the closest in meaning to the underlined word or phrase.
Ex : Hurricanes occur frequently in the Caribbean
All four choices make a meaningful sentence, but only one (b. often) keeps the meaning of the word frequently ; only often is a synonym of frequently.