"transparent" can mean both "obvious" and "hidden"
"aught" can mean both "all" and "nothing"
Is there a term for a word which has multiple definitions that are antonyms?
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Great question. These wacky words are called "contronyms." As defined by Oxford dictionaries,
"A word with two opposite meanings, e.g., sanction (which can mean both ‘a penalty for disobeying a law’ and ‘official permission or approval for an action’)."
They are often referred to as Janus words, after the Roman god Janus, whom Wikipedia describes as "the god of beginnings and transitions, and thereby of gates, doors, passages, endings and time. He is usually depicted as having two faces, since he looks to the future and to the past".