Contronyms are words that have two meanings that are the opposite of each other.
Sanction, for example, may mean restricting something (e.g. sanction against imports) or approving something (e.g. sanctioning imports).
Scan may mean to look at cursorily (e.g. scan a document) or look at in detail (e.g. scan an X-Ray)
Fine may mean excellent (e.g. fine wine) or average (e.g. the wine’s fine).
I enjoyed this list of 75 contronyms.