What does a 'protractor' measure?

A protractor is an instrument used to construct and measure plane angles. The simplest protractor comprises a semicircular disk graduated in degrees—from 0° to 180°. It is an ancient device that was already in use during the 13th century. A more complex form of a protractor, designed for plotting a ship's position on navigational charts, was invented in 1801 by Joseph Huddart, a U.S. naval captain. This instrument called a three-arm protractor, is composed of a circular scale connected to three arms. The center arm is fixed, while the outer two are rotatable, capable of being set at any angle relative to the center.

Source: Britannica