What is Allergen?

Legal Definition
An allergen is a type of antigen that produces an abnormally vigorous immune response in which the immune system fights off a perceived threat that would otherwise be harmless to the body. Such reactions are called allergies.

In technical terms, an allergen is an antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals through Immunoglobulin E (IgE) responses. Most humans mount significant Immunoglobulin E responses only as a defense against parasitic infections. However, some individuals may respond to many common environmental antigens. This hereditary predisposition is called atopy. In atopic individuals, non-parasitic antigens stimulate inappropriate IgE production, leading to type I hypersensitivity.

Sensitivities vary widely from one person (or other animal) to another. A very broad range of substances can be allergens to sensitive individuals.
-- Wikipedia
Legal Definition
Anything that causes an allergic reaction. A person must be allergic to the substance to hav a reaction. The body fights this foreign substance. AKA antigen.