An enemy is the opposite of a friend. It may also refer to two antagonistic concepts, like when craziness is said to be the enemy of reason. When someone is the enemy of someone else, the first person wants to harm the other. There are several expressions such as sworn enemy, arch-enemy, or public enemy for instance.

When two countries are into a conflict, or in war period, they are usually said to be enemies. When a soldier, or officer in charge turns his back on his allies and join the other camp, he is said to turn into an enemy.

This word can also be used when dealing with sports, when two athletes, two teams, two clubs are serious rivals, they may be said to be enemies. It is the case for instance between the two soccer teams Real Madrid and FC Barcelona in Spain. There is also the Eagles-Giants rivalry opposing the football teams Philadelphia Eagles to New York Giants, and many others...

In fictional works, heroes often have enemies. For that matter, heroes are called 'hero' and their enemies are called 'villain'. Villains are often the self-starter of the action, they disrupt the hero's life and brings the action. This kind of opposition is very regularly found in US comics, such as Batman and the Joker, Superman and Lex Luthor, Spiderman and Green Goblin...