Hyperhidrosis: A condition characterized by extreme and excessive sweating. Primary hyperhidrosis affects the hands, feet, and armpits and often has no identifiable cause. If the sweating occurs as a result of another medical condition, it is called secondary hyperhidrosis. In secondary hyperhidrosis, the sweating may be all over the body or may be localized to one area. A number of medical conditions can cause secondary hyperhidrosis.