Detestation is a feeling of disgust and loathing, like the detestation everyone in your environmental club feels toward people who litter or waste water.
When you truly, fiercely hate something or someone, that's detestation. In other words, you detest that hated thing or person. Members of a pacifist group share a detestation for war, and a criminal mastermind might feel nothing but detestation for the superhero who keeps thwarting her plans. The word comes from the Latin verb detestari, "to curse or express abhorrence for."