The swimming stroke where both arms move simultaneously in a circular motion is the butterfly stroke. In butterfly, the arms recover together over the water and then pull simultaneously beneath the surface, while the legs perform a distinctive dolphin kick.
The stroke developed from breaststroke experimentation in the 1930s. Swimmers discovered that recovering both arms over the water could produce greater speed, and butterfly was eventually recognized as a separate stroke by swimming authorities in the 1950s. It made its Olympic debut as an independent event at the 1956 Melbourne Games.
A common mix-up is confusing butterfly with breaststroke because both use simultaneous arm movements. Breaststroke has a frog-style kick and arms that recover beneath the water, whereas butterfly uses the dolphin kick and an over-water arm recovery. Freestyle is not a single prescribed stroke in competition, although front crawl is almost always used.