In lexical semantics, opposites are words that lie in an inherently incompatible binary relationship as in the opposite pairs male : female, long : short, up : down, and precede : follow. The notion of incompatibility here refers to fact that one word in an opposite pair entails that it is not the other pair member. For example, something that is long entails that it is not short. It is referred to as a 'binary' relationship because there are two members in a set of opposites. The relationship between opposites is known as opposition. A member of a pair of opposites can generally be determined by the question What is the opposite of X ?
The term antonym (and the related antonymy) has also been commonly used as a term that is synonymous with opposite; however, the term also has other more restricted meanings. One usage has antonym referring to both gradable opposites, such as long : short, and (non-gradable) complementary opposites, such as male : female, while opposites of the types up : down and precede : follow are excluded from the definition. A third usage (particularly that of the influential Lyons 1968, 1977) defines the term antonym as referring to only gradable opposites (the long : short type) while the other types are referred to with different terms. Therefore, as Crystal (2003) warns, the terms antonymy and antonym should be regarded with care. In this article, the usage of Lyons (1963, 1977) and Cruse (1986, 2004) will be followed where antonym is restricted to gradable opposites and opposite is used as the general term referring to any of the subtypes discussed below.
Opposites are interestingly simultaneously different and similar in meaning. Typically, they differ in only one dimension of meaning, but are similar in most other respects, including similarity in grammar and positions of semantic abnormality. Additionally, not all words have an opposite. Some words are non-opposable. For example, the word platypus has no word that stands in opposition to it (hence, the unanswerability of What is the opposite of platypus?). The main reason is that animal or plant species have no binary opposites (other than possible gender opposites such as lion/lioness, etc.). Other words are opposable but have an accidental gap in a given language's lexicon. For example, the word devout lacks a lexical opposite, but it is fairly easy to conceptualize a parameter of devoutness where devout lies at the positive pole with a missing member at the negative pole.
Opposites may be viewed as a special type of incompatibility. Words that are incompatible create the following type of entailment (where X is a given word and Y is a different word incompatible with word X):
An example of an incompatible pair of words is cat : dog:
This incompatibility is also found in the opposite pairs fast : slow and stationary : moving, as can be seen below:
Cruse (2004) identifies some basic characteristics of opposites:
Complementary opposites are pairs that express absolute opposites, like mortal and immortal.
Antonyms, from the Greek anti ("opposite") and onoma ("name") are word pairs that are opposite in meaning, such as hot and cold, fat and skinny, and up and down. Words may have different antonyms, depending on the meaning. Both long and tall are antonyms of short. Antonyms are of four types:
Though the word antonym was only coined by philologists in the 19th century, such relationships are a fundamental part of a language, in contrast to synonyms, which are a result of history and drawing of fine distinctions, or homonyms, which are mostly etymological accidents or coincidences.
Languages often have ways of creating antonyms as an easy extension of lexicon. An example is the English prefixes in- and un-. Unreal is the antonym of real and indocile is of docile.
Some planned languages abundantly use such devices to reduce vocabulary multiplication. Esperanto has mal- (compare bona = "good" and malbona = "bad"), Damin has kuri- (tjitjuu "small", kuritjitjuu "large") and Newspeak has un- (as in ungood, "bad").