Antonyms are those words that express an opposite or contrary idea to that expressed by another word, whereas, as happens with synonyms, they must correspond in terms of grammatical category to be considered antonyms.; for instance, Vice is the antonym of virtue, of course, of dark, hate love, great when small, work leisure, day at night, young from old, admiration from contempt, high from low, among others.
Almost all the words with which we communicate present an antonym, or at least different words that soften the qualities of the object they represent a little, until finally the totally opposite term is reached. Let's see it with some examples: cold, warm, hot; high, medium, low.
If we look at a chromatic scale, we will appreciate very clearly that a wide range and variety of grays appears between black and white, so the same can be applied to the subject we just mentioned.
But not all antonyms are strongly opposed only, but there are some, such as father and son, buying and selling, that complement each other, that is, one cannot exist without the other in any way.
There are a wide variety of dictionaries specialized in this type of words, which by the way make it very easy to find them, when it is necessary in a text or conversation to find the exact opposite meaning. In addition, the knowledge of the antonyms of almost all the words allows to enrich the language and in some cases to understand much better the meaning of some words, because for example there are some that to understand specifically what happiness is, they will need to know what it is not and to they must inevitably know what suffering is.
So, there are three types of antonyms: gradual (the two words are gradually opposed, the case we mentioned cold-temperate-hot), complementary (the meaning of one overrides the other) and reciprocal (the meaning of one of the words supposes the existence of the other, one cannot be given without the other, parent-child).
When the opposite applies to people, whether they are protagonists of literary texts, films, novels, among others, they are called antagonists.