Synonyms are words which have different forms but similar meanings. The word similar is preferred because, strictly speaking, there are no words which are exactly the same in meaning. Synonyms are therefore further classified.

　 Dialectal synonyms are words which are similar in meaning but used in different dialects of the language. There are many pairs of such synonyms between British English and American English. Torch/flashlight, flat/apartment, autumn/fall, etc, are all dialectal synonyms.

　 Synonyms of different styles can be exemplified by gentleman/guy, policeman/cop, explode/blow up, etc. As formality is a gradable notion, there can be a set of synonyms which differ from very formal to very colloquial or even slangy. Pass away/ die/ kick the bucket is a synonymous set. Mismatch of expressions that belong to different styles can sound funny. If someone enters the classroom and says, “Ladies and Gentlemen, shut up”, you will be surprised or laugh.

　 Synonyms of different registers are words similar in meaning but used in different spheres. Salt and sodium chloride refer to the same substance. But no one would say at the dinner table “Would you pass me the sodium chloride?” For the same reason, one would not say in a laboratory, “Please pass me the salt.” Similarly, thin out is an expression used in gardening or farming, while thin down is one in hairdressing.

　 Synonyms differing in affective meaning are words that carry the speaker's positive or negative attitude. Both beauty and siren refer to a good-looking woman, but the latter implies the speaker's negative evaluation of the person's morality.

　 Synonyms differing in collocation are words which are similar in meaning but co-occur with different words. Assassinate and murder refer to killing a person deliberately, but they are different in collocation. The former requires an object that denotes a very important person and the verb implies a political motivation of the action; the latter is followed by a noun or pronoun that denotes an ordinary person and the action is not for political purpose.

　 Synonyms are frequently used in speaking and writing as a cohesive device. In order to avoid repetition the writer/speaker needs to use a synonym to replace a word used in the previous co-text when he/she wants to continue to address that idea. “I took leave, and turned to the ascent of the peak. The climb was perfectly easy.” (Halliday and Hasan 1976: 278). The two words in italics are synonyms. They together function to create cohesion of the text.