A pointed arch with four arcs, the centers of two inside the arch, of the other two outside; this produces a compound curve of two parts, one convex and the other concave.
a pointed arch having an S-shape on both sides
((English) Architectural term for arches formed by two S-shaped curves, with a pointed top. Most typically the arched form is convex towards the top, concave towards the bottom. Islamic architects often used this form.
an arch with a compound curve, partly concave and partly convex; looks like the keel of a boat. (Also known as a KEEL arch.) (IMAGE)
It is a pointed arch flattened appeared firstly in Muslim Persia. It was widely used in India and later in Europe in late Gothic.