The term SEMICIRCLE SHAPES can be summarized as ARCS, which is a four-letter word. An arc is a curved line that forms a part of the circumference of a circle. Arcs are often used to create semicircles, which are half circles. A semicircle is a two-dimensional shape that is made up of two equal parts, with one part being a straight line and the other part being a curved line. Arcs are also used in many other shapes, such as ovals, ellipses, and polygons.