An enemy or foe was someone who would be hostile as opposed to a friend or ally.
In 2259, La'an Noonien-Singh noted that the Federation taught that if a way was found to empathize with an enemy then one day they could become friends. She considered this wrong, because she believed some things in the universe, such as the Gorn, were evil. (SNW: "Memento Mori")
In 2267, Doctor Leonard McCoy asked Captain James T. Kirk, "Does Maab know that the Klingons are our sworn enemies by their own words?". (TOS: "Friday's Child")