an enemy: one who is actively hostile to another person, a people, or God
This is the Hebrew word for "enemy" in virtually all its forms, the person who seeks your harm, the army arrayed against your nation, or the power that stands against God. It comes from a verb meaning "to be hostile" and functions in Hebrew as a noun, but the active sense is still felt: this is not merely someone you dislike, but one who is actively working against you.
The word spans three contexts without changing its core meaning. In military and national settings it names the armies and nations that fight against Israel, from the conquest narratives through the exile prophets. In personal and lament poetry it names the individual rival or pursuer who threatens the speaker's life or honor. In theological contexts it names those who stand in opposition to God himself, a use especially prominent in the Psalms and the prophets. Across all three, the basic idea is the same: one who is set against.
What distinguishes אֹיֵב from its near-synonyms is its breadth and its frequency. It is the ordinary, unmarked Hebrew word for enemy, the one a reader will encounter most. Unlike שׂוֹנֵא (one who hates), it does not emphasize an internal emotion of hatred; unlike צַר (foe, one who is in tight opposition or distress), it does not carry the spatial or psychological image of constriction. It simply names the adversarial relationship as a fact.