1.NounA disorder of the immune system causing adverse reactions to substances (allergens) not harmful to most and marked by the body's production of histamines and associated with atopy, anaphylaxis, and asthma; any condition of hypersensitivity to a substance.
"My dad can't live with pets because of an allergy."
2.NounSpecifically, hypersensitivity of class I in the modern classification thereof: the immunoglobulin E–mediated type.
3.NounAn antipathy, as toward a person or activity.