Antonyms are words of the same part of speech which have contrasting denotational components of meaning,
e.g. hot - cold, to accept - to reject, up - down.
So, antonymy is "oppositeness of meaning".
There are different kinds of oppositeness.
1. Some antonyms are contradictories, eg. dead - alive, single - married. One term contradicts the other, and to use not before one is to make it equivalent to the other,
e.g. If Peter is married, he is not single.
If he is single, he is not married.
Such words can be treated in term of complementarity, the items being complementary to each other, because there are only two possibilities.
The negative prefixes in-, im-, dis-, un- are often attached to words to give them the opposite meaning,
e.g. possible - impossible, lucky - unlucky, like - dislike.
(But there are exceptions: impassive, invaluable, inflammable are not the opposites of passive, valuable, flammable, but are almost synonymous, e.g. invaluable "so valuable that it's impossible to estimate the value".)
2. The second group of antonyms includes contraries. They should be treated in term of gradability, i.e. the degree of a quality they denote. So these words are gradable, placed on a graded scale for comparison,
e.g. wide - narrow:
A is wider than B.
B is narrower than A.
A is less narrow than B.
B is less wide than A.
Since such antonyms are gradable, there are often intermediate terms. Thus, we have not just hot - cold, but hot - warm - cool - cold, with the intermediate warm and cool forming a pair of antonyms themselves.
In each pair one of the terms is marked and the other is unmarked, and only the latter is used to ask about or describe the degree of the gradable quality,
e.g. How high/wide is it? But not: How low/narrow is it?
It is 3 feet high. But not: It is 3 feet low.
In English, the "larger" term seems to be unmarked.
Unlike contradictories, with the gradable antonyms, to say that something is not, for instance wide isn't to say it is narrow (and v.v.). The possibility of being neither wide nor narrow is left open.
There is no absolute distinction between the two abovementioned types of antonyms. We can treat male/female, married/single, dead/alive as gradable antonyms on occasion,
e.g. He is more dead than alive.
On the other hand, there are gradable antonyms, e.g. honest - dishonest, obedient - disobedient, open - shut, in which the denial of one is taken to assert the other, thus, though we may say, "Bill is more/less honest than John", "Bill isn't honest" implies that he is dishonest.
3. A quite different kind of oppositeness is found with pairs of words which exhibit the reversal of a relationship between items,
e.g. buy - sell, husband - wife.
If A sells to B, B buys from A.
If A is B's husband,B is A's wife.
These are relational opposites. The relationship between them can also be called converseness, and the words are convertibles: lend - borrow, rent - let, own - belong to, give - receive, above - below, in front of - behind, parent-child, etc.
It is worth noting that "true" gradable antonyms can be treated basically in terms of relational opposites,
e.g. If A is wider than B, B is narrower than A.
4. Incompatible terms, e.g. morning, night, evening, day, or blue, red, green, etc, are not true antonyms, as they are characterized by the semantic relation of exclusion, not of contradiction, i.e. to say "It was morning" excludes "It was evening/day/night," and negation of one item doesn't imply semantic equivalence with the others, e.g. It wasn't night. ≠ It was day.
Most antonyms are adjectives, e.g. high - low, strong - weak, old - young, etc. because qualities are easily graded and contrasted. Verbs take second place, e.g. lose - find, open - close, live - die. Nouns are not very rich in antonyms, e.g. good - evil, friend - enemy, joy - sorrow. Antonymic adverbs can be adverbs proper, e.g. up - down, ever - never, now - then, here - there, or they can be derived from antonymic adjectives, e.g. warmly - coldly, merrily - sadly.
Like synonyms, antonyms are not words but lexico-semantic variants. A polysemantic word may have an antonym (or several antonyms) for each of its meanings,
e.g. dull ¹ "deficient in interest" - amusing, interesting, entertaining,
dull ² "deficient in intellect" - clever, bright, capable.
Together with synonyms, antonyms represent the most important expressive means of the language. They are used by authors as a stylistic device of contrast,
e.g. The writer should ... care nothing for praise or censure, failure or success." (W.S.Maugham)
Antonyms are used in numerous proverbs and sayings,
e.g. Faults are thick where love is thin.
Joy and sorrow are like today and tomorrow.
