An enemy is an entity that is forcefully adverse to the person denoting it "enemy". The term is usually used in war, as a denomination of the side one is fighting. It may also be used about weather, diseases and a host of other things, the common connection being that they may cause injury or even death to those they "oppose".

Enemies usually (or perhaps always) have conflicting, or directly opposed, objectives and goals - this state of conflict is typically why two sides regard each other as enemies.

"Enemy" is a strong word, evoking associations of hate, violence, battle and war.

Quotes:

The enemy of my enemy is my friend.

The enemy of the enemy of my enemy is my enemy.