Lap Versus Length
Steve Tarpinian explains in "The Essential Swimmer" that in the sporting world, laps are considered one round trip, such as one full circle around a track. Therefore, one lap in a pool is usually referred to as two lengths, or to there and back. The terminology of laps and lengths can be used incorrectly by novice swimmers, who often refer to a lap as one length of the pool.
Short-course swimming pools in the United States are usually either 25 yards or 25 meters in length and are often found in recreational facilities and high schools. One full lap in a short-course pool is equal to 50 yards or 50 meters, depending on the particular pool. Pools used for higher levels of competition, such as by collegiate and professional swimmers, are called long-course or Olympic-size pools and run 50 meters in length. Therefore, one lap in a long-course pool is 100 meters.