butterfly door

plural butterfly doors

1. automotiveA 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.
2. A door consisting of two leaves that swing on a pivot above the center of the firebox opening to open and close.

   The two-hopper, self-emptying ashpan has butterfly doors operated by a hand lever from ground level, and its damper doors, fore and aft, are controlled separately by standard hand wheels and screw gear.

   1953 February, “British Railways Standard Class "4" 2-6-0 Locomotives”, in Railway Magazine, page 108

3. architectureA 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.