Athlete's foot: A skin infection caused by a fungus called Trichophyton that thrives within the upper layer of the skin when it is moist, warm, and irritated. The fungus can be found on floors and in socks and clothing, and it can be spread from person to person through contact with these objects. However, without proper growing conditions, athlete's foot fungus will not infect the skin. It can be treated with topical antifungal preparations. Also known as tinea pedis, athlete's foot is a form of ringworm.