Atopy is the medical term for allergy; more specifically allergic reactions in the body resulting from inhaled or contact allergens such as pollen, mold or dust. The word atopy comes from the greek word "atopia" meaning extraordinariness, and equivalent to "atopos" meaning out of place or unusual.1
Canine atopy is similar to hay fever in humans, but instead of causing runny nose / eyes and sneezing, the vast majority of veterinary atopy cases result in itchy skin. Some animals do suffer from asthma or related respiratory signs, but they are in the minority. Atopy is thought to be heredity, so breeding atopic animals is discouraged.
To learn more about atopy, please see the FAQ titled "Why does my pet itch and scratch so much?"
Reference1: "atopy." Dictionary.com Unabridged (v 1.1). Random House, Inc. 09 Jul. 2007.
<Dictionary.com http://dictionary.reference.com/browse/atopy>.
