Half a circle is called a "semicircle" in geometry.
http://wiki.answers.com/Q/What_is_a_half_of_a_circ...

A half circle protractor measures angles when its straight edge is aligned with one side of the angle. Since angle must measure less than 360 degrees, the shape of a half circle protractor
http://www.ehow.com/facts_7227568_halfcircleprot...

The radius of a circle is the distance from the center of a
http://www.chacha.com/question/whatishalfofthe...

An annulus. Area = pi (R 2  r 2 ) when R is radius of larger circle and r is radius of smaller circle.
http://wiki.answers.com/Q/What_is_a_circle_with_a_...
