A semicircle is a two-dimensional geometric object consisting of an arc and a segment that together enclose an area in the shape of half a circle.

The radius of a semicircle is defined as the distance from the midpoint of the segment to any point of the arc.

The perimeter and the area of a semicircle with the radius $r$ can be calculated using the following formulas. $$\begin{array}{rl}\text{Perimeter}& P=\pi r+2r\\ \text{Area}& A=\frac{1}{2}\pi r^2\end{array}$$