enemy
adversary / foe
[C] Countable
pl: enemies
The word carries a heavy weight of active hostility and mutual antagonism. It implies a relationship defined by conflict, whether that conflict is a violent war or a cold, psychological rivalry. It is far more intense than a dislike or a disagreement, suggesting a desire to see the other party fail or suffer.
In modern usage, the term is often applied to internal struggles, such as the enemy within, referring to self-sabotage or guilt. It can also be used ironically among friends to describe a playful rivalry in sports or games, though its primary connotation remains one of danger and opposition.
Countable when referring to a specific person or group you are fighting against.