What Is A Norman Arch?

It's the round one.  It's a distinctive feature of Norman architecture, which itself forms an architectural style of its own.  The Norman arch itself is open and entirely round, whereas the medieval and Gothic styles that preceded/followed the Norman arch were pointed at the top, and much more narrow in the case of older style architecture.

Norman architecture was first described as a distinct style in the early 1800s.  Norman buildings can still be seen in England, Scotland, Ireland, France and Sicily.

The Norman influence on architecture was present in Britain before the 1066 invasion, but out of date by the year 1200.  it was replaced with Gothic arches:  large, and very pointed at the top.  Why?  The pointed top was thought to aim people's attention better towards heavens and spiritual matters, and away from the earthly things.  That is a main reason why Gothic arches became more popular within churches.