Round-topped openings—typically just called an “arch”—come in two main styles: a “segmented arch” and an ellipse.
A segmented arch, like this one from the Washington, DC Brewmaster’s Castle built in the Richardsonian Romanesque style, is a segment of a big circle: if you projected the lines of the arch around it would make a circle.
An ellipse is really three separate radii, which combine to form a “flatter” top, sort of like an egg sliced in half.
