an enemy: someone or something set in hostility against another
At its core this word names an enemy, the person (or power) who stands against you rather than with you. It began as an adjective meaning hostile or hated and settled into a noun for the hostile one, the adversary. The New Testament stretches it across every level of opposition, from the neighbor who wishes you harm to the cosmic forces arrayed against God.
Most often it is an ordinary human enemy: the person you are commanded to love, the household member turned against you, the armies that surround a city. In this everyday use the word carries no special weight, only the plain fact of hostility.
A second circle of uses aims the word at God. People can be enemies of God, of the cross, of righteousness, alienated and opposed until they are reconciled. The Psalm that promises a king his enemies as a footstool (Psalm 110:1) is quoted again and again to describe every power that resists God's rule being finally subdued.
At its sharpest the word becomes almost a title. The enemy who sows weeds among the wheat is the devil himself, and Jesus speaks of the power of the enemy set against his followers. Paul even personifies death as the last enemy, the final hostile power to be destroyed. So one small word runs from the quarrel next door to the last thing that must fall before God's victory is complete.