An "allergy" is a sensitivity to something that is ordinarily harmless. When your body tries to get rid of the "allergen" (foreign substance), you experience symptoms like sneezing, watery eyes, and a runny, stuffy, or itchy nose. Typical allergens include pet dander, pollen, dust mites and mold.

The medical term for nasal allergy symptoms is "allergic rhinitis." ("Rhinitis" comes from the ancient Greek word "rhinos" for nose, and "itis" means inflammation.)

"Seasonal allergic rhinitis" describes nasal allergies that change with the seasons due to pollen from plants. For these people, symptoms come and go with the pollination seasons of certain trees, grasses or weeds. Pollen levels from these plants can vary day to day, depending upon several factors, including the weather. High pollen levels can, in turn, affect the severity of symptoms.

"Hay fever," a popular term for seasonal allergies, was coined in 1828 by a British physician, Dr. John Bostock, who noticed that his symptoms worsened during the British haying season. (The expression is misleading, because allergies seldom cause a fever and are rarely related to hay.)

"Perennial (or chronic) rhinitis" describes nasal allergies that can occur any time of the year, which are caused by substances like dust mites, mold spores, cockroaches or pet dander.