The semantic relation that holds between two words that can (in a given context) express opposite meanings.
Near or precise oppositeness between or among words. Good and evil are antonyms, and therefore evil and bad are synonyms in this context.
Antonymy may vary with context and situation: in discussing a color, dull and bright are antonymous, but when talking about knives and blades, the opposite of dull is sharp (when it comes to intellect, bright and sharp are equally used).