The noun contronym (also contranym) denotes a word with two opposite or contradictory meanings.

Contronyms can take the form:
– of polysemous words—for example sanction, which can denote both a penalty for disobeying a law and official permission or approval for an action;
– of homographs—for example let (from Old English lǣtan), meaning to permit, and let (from Old English lettan), meaning to hinder.

Modelled on nouns such as synonym and antonym, contronym is from the prefix contra-, denoting opposition, and the combining form -onym.
—Cf. also aptonym and backronym.

The noun contronym was coined by Jack Herring in the February 1962 issue of Merriam-Webster’s magazine, Word Study—the following is from the Chattanooga Daily Times (Chattanooga, Tennessee) of Thursday 8^th February 1962:

And Now ‘Contronyms’

Most of us are familiar with synonyms and antonyms. What about “contronyms”?
Jack Herring of Arizona State University suggests the name for any word which is used in two senses seeming to contradict each other.
He cites several examples, each with two definitions which in a sense are contradictory. Among them:
Fast: (1) firmly fixed; stable, unyielding.
(2) moving or able to move rapidly.
Scan: (1) to examine point by point; scrutinize.
(2) to look over hastily (colloq.).
Phenomenon: (1) any observable fact or event.
(2) an exceptional or abnormal person, thing or occurrence.
Cleave: (1) to adhere closely; to stick; cling.
(2) to part, divide; to split; crack; separate.
Words are the essential building blocks for the structure of communication. Some, like some building materials, have two faces startling in contrast. The craftsman knows how to use such “contronyms”—and synonyms and antonyms and all the rest, as well—so that what he says or writes makes a pleasing whole, its meaning clear.

Joseph Twadell Shipley (1893-1988) had developed the same notion in Playing With Words (Englewood Cliffs (New Jersey): Prentice-Hall, Inc., 1960). He called a word with two opposite or contradictory meanings autantonym, from the prefix auto-, meaning self, and the noun antonym:

AUTANTONYMS

Opposites in meaning are called antonyms. But you have heard that extremes meet.
Hence it may be said that to shout and to whine, to bluster and to cringe, to flatter and to vilify, are opposite sides of the same coin. The man that does one, turned around, will do the other.
Without even turning around, some words, because of the curious growth of our language, contradict themselves. They are self-opposites, or autantonyms. Here are a few of them:
let: to allow; to hinder, as a let ball in tennis
cleave: to cut clean apart; to cling tight together
fast: a fast horse runs; a fast color doesn’t run
dust: a coat, or a field of crops with insecticide
trim: a fat cut of meat, or a Christmas tree
tripping: stumbling; but moving deftly, as when one trips the light fantastic
nervous: a nervous style has “nerve”; a nervous person lacks it
temper: to harden steel; to soften the wind to the shorn lamb
mortal: producing death; subject to death
weather: to wear well; to wear out
to think better of a person is to admire him more; to think better of a proposed action is to like it less and turn from it
wind up a clock, it starts; wind up a business, it stops
counterpart
overlook
cavalier
unbending