Butterfly swords are a commonly used name for the Chinese húdiédāo (蝴蝶刀), a type of double swords that seem to have originated in or around the port cities of southern China.
They are also known in the local Cantonese dialect as bat jam do (八斬刀) or "eight cutting knives", pronounced bāzhǎndāo in Mandarin.
Their defining feature is a large D-shaped guard with upturned quillon at the back, usually of a copper alloy but sometimes made of iron. They typically have half-hilts that fit in a single scabbard side-by-side and can be drawn as if they were one weapon. The blades are usually quite straight and with a fairly strong taper.
For more information, see: húdiédāo (蝴蝶刀)