A New Dictionary

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."
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
cleave - separate, adhere
clip - fasten, detach
custom - usual, special
deceptively smart - smarter than one appears, dumber than one appears
dike - wall, ditch
dust - add fine particles, remove fine particles
enjoin - prescribe, prohibit
fast - quick, unmoving
first degree - most severe (e.g., murder), least severe (e.g., burn)
give out - produce, stop production
grade - incline, level
handicap - advantage, disadvantage
left - remaining, departed from
liege - sovereign lord, loyal subject
mean - average, excellent (e.g., "plays a mean game")
out of - outside, inside (e.g., "work out of one's home")
pitted - with the pit in, with the pit removed
put out - extinguish, generate (e.g., something putting out light)
quite - rather, completely
rent - buy use of, sell use of
sanction - approve, boycott
screen - show, hide
strike - hit, miss (in baseball)
transparent - invisible, obvious
unbending - rigid, relaxing
wind up - end, start up (e.g., a watch)
with - alongside, against

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