A protractor is a flat school tool, usually clear plastic, in the shape of a half-circle. You use it to measure an angle in degrees, or to draw an angle of the size you want. The curved edge is marked from 0 to 180. Some are a full circle, marked to 360, but the half-circle is the one most students get. This is classroom English, not a fancy word. Though actually, doctors also use protractor for a muscle that pulls a body part forward — but in school it almost always means the plastic half-circle.