What term describes a deep-seated, often mutual hatred?

The term that describes a deep-seated, often mutual hatred is "enmity." This word indicates a more entrenched and pervasive animosity between parties, often suggesting a long-standing and reciprocal nature of the hostility. Enmity goes beyond individual acts of aggression or dislike; it encapsulates an enduring state of ill will and opposition that typically exists between groups or individuals.

In contrast, "hostility" refers more broadly to unfriendliness or opposition but lacks the depth of mutual hatred implied by enmity. "Antipathy" denotes aversion or strong dislike, which might not always convey the intensity and mutual aspect found in enmity. "Bitterness" often expresses a feeling of resentment or anger, typically focused on a particular grievance, which may not encapsulate the broader sense of lasting hatred that enmity conveys. Thus, "enmity" is the most precise term for describing a deep-seated and mutual animosity.