Noun
(automotive) A car door similar to scissor door, but while scissor doors move straight up via hinge points at the bottom of the A-pillar, butterfly doors move up and out via hinges along the A-pillar.
(of stationary or locomotive boilers) A door consisting of two leaves that swing on a pivot above the center of the firebox opening to open and close.
(architecture) A door which pivots on a central axis, used in locations with a pressure differential between the two sides of the door, so that forces exhibited do not inhibit opening and closing of the door, due to contrary action of forces on the two halves of the door which open inwards and outwards at the same time.
Source: en.wiktionary.org