What's that embossed or raised part of an arc or arch called? I am looking for the upper part of the shape, which is kind of raised and forms a mini circle.
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I don't see a "circular" bit at the top of the arch in the OP's picture.
The parts of the arch are:
Perhaps ogive, of which wikipedia says:
An ogive (/ˈoʊdʒaɪv/OH-jyv) is the roundly tapered end of a two-dimensional or three-dimensional object.
Later, the wikipedia article on ogives distinguishes between "spitzer ogives" and "elliptical ogives". A Gothic arch typically is a spitzer ogive, while the Gateway Arch illustrated in the question is a "weighted or flattened" catenary arch. A catenary, wikipedia says, is "superficially similar in appearance to a parabola (though mathematically quite different)". At certain magnifications, portions of parabolas, ellipses, and catenaries look much alike, and I think the top of the Gateway Arch looks a lot like an elliptical ogive.
Wiktionary lists several senses of ogive, including "A Gothic pointed arch, or a rib of a Gothic vault" and "(ballistics) The pointed, curved nose of a bullet, missile, or rocket".
You may also find terms like the following of interest: archivolt (or voussure), "an ornamental molding or band following the curve on the underside of an arch"; spandrel; intrados, "inner curve of an arch or vault"; voussoir, "a wedge-shaped element, typically a stone, used in building an arch or vault". See nomenclature in first diagram in wikipedia arch article.