A semi-circle (or semicircle) is simply one-half of circle.
It can also be thought of as a sector with an angle of 180 degrees.
If the perimeter of the semi-circle is needed, add together the lengths of
one-half of the circumference + diameter
Where (for brevity) it says 'radius', 'circumference' and so on, it should, more correctly, be something like 'length of radius' or 'size of circumference' etc.