Definition
Semicircle
A semicircle is half of a circular region, made by cutting along a diameter.
What it means
The cut must pass through the circle’s centre and reach both sides. That straight cut is the diameter; the other edge curves halfway around the circle.
Remember it
One straight diameter, one half-round arc.
Example
A paper circle is cut along a 10 cm line that passes through its centre. The cut is a diameter, so each piece is a semicircle and each piece has a 10 cm straight edge.
Watch for
A line that misses the centre cuts off a circular segment, not a semicircle.
Common mix-up
The curved arc is only part of the boundary; a semicircle also has the straight diameter.
Exact meaning
A semicircle is one of the two equal halves of a circular region formed by a diameter. Its boundary consists of that diameter and a 180° arc.