(formal) An evil or wrongdoing.

plural of malum

(law) An action that is not inherently evil, but is nevertheless illegal only because prohibited, as opposed to malum in se. A malum prohibitum offense is something that is wrong only because a statute makes it so, or by consensus that society agrees to prohibit the act, and is typically regulatory in nature and often result in no direct injury or danger to the person, entity, or property but only merely create the danger or probability of it which the statute attempts to minimize. Used to develop consensual crimes.

(literally) A wrong in itself; that which is injurious in its consequence.