a street name for the drug amyl nitrate. some people call them "poppers" because of the old-style glass vial that one would "pop" underneath their nose.
rush is classified as an inhalant and is commonly associated with the gay community, as it gained it's notoriety for relaxing the anal muscles while not adversely affecting an erection. generally, it produces a feeling of blood "rushing" to the brain along with mild euphoria.
amyl nitrate can be used to treat angina pectoris. "..and one for the doctor..."