An antonym is a word whose meaning is directly opposite to another word's meaning. In this thesaurus, an antonym is a word that has a meaning that completely cancels out another words meaning. Short and tall are complete opposites. Something cannot be both short and tall at the same time, and both words suggest about the same degree of difference from the norm or average in height. Good and evil are another pair of exact opposites. Logically, something cannot be both good and evil in the same way and at the same time.

Words that are only opposite in some aspect of their meaning cannot be said to be true antonyms. For example, sad, which means "causing unhappiness," is not a true antonym of funny, which means "causing or intended to cause laughter." The opposite of unhappiness is happiness, and so the opposite of sad is happy. There are things, like a frowning clown, that can be both funny and sad, so these words can't be antonyms. Pairs of words like sad and funny are better regarded as near antonyms.

The words in the antonym lists are the exact opposites of the headword and the words in the synonym lists. Not every synonym group has antonyms, and antonym lists tend to be shorter than near antonym lists. Antonyms are listed alphabetically, just as synonyms are. They appear as boldface headwords at their own alphabetical places in the thesaurus only if they are part of a synonym group elsewhere.

Ordinarily, all the words in an antonym list are synonyms of each other. Sometimes, however, a semicolon in an antonym list divides words which, while they are exact opposites of the words in the synonym group, are not synonyms of each other. Here is an example:

Word Entry: present
Function: adjective
Text: 1 existing or in progress right now <I am very busy at the present moment>
Synonyms: current, extant, ongoing, present-day
Antonyms: ago, past; future

The opposite of "existing or in progress right now" is "not existing or in progress right now." All three of the words in the antonyms list fit that meaning. Ago and past, separated by a comma, are synonyms of each other; they both can mean "earlier than the present time." But ago and past are not synonyms of future, which means, of course, "occurring at a later time."