Learner's definition of ENEMY
1 : someone who hates another: someone who attacks or tries to harm another
◊ If you are your own worst enemy you act in a way that causes harm to yourself or to the people or things that you care about.
2 : something that harms or threatens someone or something
Tradition is the enemy of progress.
In many countries today, drug abuse is public enemy number one. [=the most dangerous threat to society]
3 a : a group of people (such as a nation) against whom another group is fighting a war— usually singular
b : a military force, a ship, or a person belonging to the other side in a war— usually singular
Some of the soldiers went over to the enemy.
He found himself behind enemy lines.
The plane was shot down by enemy fire.