What is the definition or description of: allergy?

Not a simple answer: An allergy is an exagerated immune responce to a typically benign trigger. You can have allergic responces to many things like pollens, animal dander, foods, medications, among other things. Symptoms may be bothersome, but benign, or can be fatal. Not all reactions to exposures are allergies and if you think you may be allergic , i recommend being evaluated by a board certified allergist.