In lexical semantics, opposites are words lying in an inherently incompatible binary relationship. For example, something that is even entails that it is not odd. 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) is commonly taken to be synonymous with opposite, but antonym also has other more restricted meanings. Graded (or gradable) antonyms are word pairs whose meanings are opposite and which lie on a continuous spectrum (hot, cold). Complementary antonyms are word pairs whose meanings are opposite but whose meanings do not lie on a continuous spectrum (push, pull). Relational antonyms are word pairs where opposite makes sense only in the context of the relationship between the two meanings (teacher, pupil). These more restricted meanings may not apply in all scholarly contexts, with Lyons (1968, 1977) defining antonym to mean gradable antonyms, and Crystal (2003) warning that antonymy and antonym should be regarded with care.

Opposition is a semantic relation in which one word has a sense or meaning that negates or, in terms of a scale, is distant from a related word. Some words lack a lexical opposite due to an accidental gap in the language's lexicon. For instance, while the word "devout" has no direct opposite, it is easy to conceptualize a scale of devoutness, where "devout" lies at the positive end with a missing counterpart at the negative end. In certain cases, opposites can be formed with prefixes like "un-" or "non-," with varying levels of naturalness. For example, "undevout" is found in Webster's 1828 dictionary, while the prefix pattern of "non-person" could theoretically extend to "non-platypus."

Conversely, some words appear to be derived from a prefix suggesting opposition, yet the root term does not exist. An example is "inept," which seems to be "in-" + *"ept," although the word "ept" itself does not exist^{[citation needed]}. Such words are known as unpaired words. 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:

It's fast  entails  It's not slow

Cruse (2004) identifies some basic characteristics of opposites: