In layman’s terms, a ‘lap’ signifies a full round or completion of a course. In pool lingo, the course is the length of the pool, from one end to the other. So a ‘lap’ here is one full length of the pool, varying according to the pool size. In an Olympic-sized pool, a lap would be 164 feet, whereas, in an American short-course pool, a lap would be 25 yards.