What is a contronym?

We all know what synonyms, homonyms and antonyms are, but what is a contronym?

Contronyms are words who have two opposite or contradictory meanings.

For example, sanction means both a penalty for disobeying a law and official permission or approval for an action.

Other interesting contronyms are:

consult both mean to give advice and to get advice

throw out could be to dispose of, or to propose consideration of (an idea)

left means gone or remaining behind

lease: to rent a property or to offer a property for rent

seed: to put seeds (in the ground) or to remove seeds (from a fruit)

apology: an expression of regret about something or a defense of something 

There are a lot more contronyms in English. Check out this list for more examples.