A synonym is a word that means the same as another. Necessary and required are synonyms. An antonym is a word that means the opposite of another. Wet and dry are antonyms. While synonyms and antonyms are not in themselves interesting, the complexities and irregularities of the English language sometimes make synonyms and antonyms interesting to explore. Many complexities result from words having multiple definitions. A trivial example is a word with synonyms that aren't synonyms of each other, the word beam, for example, having the synonyms bar and shine. Similarly, some words have antonyms that are neither synonyms nor antonyms of each other but completely unrelated: the word right, for example, having the antonyms wrong and left.
A more interesting paradox occurs with the word groom, which does not really have an antonym in the strictest sense but has an opposite of sorts in the word bride, which can be used as a prefix to create a synonym, bridegroom.
The word contronym (also the synonym antagonym) is used to refer to words that, by some freak of language evolution, are their own antonyms. Both contronym and antagonym are neologisms; however, there is no alternative term that is more established in the English language.
Contronyms are special cases of homographs (two words with the same spelling). Some examples:
- anabasis - military advance, military retreat
- apology - admission of fault in what you think, say, or do; formal defense of what you think, say, or do
- aught - all, nothing
- bolt - secure, run away
More contronyms at http://www.rinkworks.com/words/contronyms.shtml