The etymology of the term compass derives from the ancient Greek word pyxis, which means wooden box. In nautical colloquiallism, it is also called bussulas from the Italian word bussola and more seldomly compasso (cf. L passus, pace; OF compas) from the English word compass. It is an instrument with which the orientation of the user is achieved. The compass in navigation constitutes the most important nautical aid, with which both the courses of the ships as well as their navigational bearings are plotted and observed.

Due to the fact that the instrument was developed out of necessity in shipping but also due to its importance, it is more commonly called the nautical compass.

Today the nautical compass is distinguished into, on the one hand, the magnetic compass, which is based on the function of the magnetic needle and it is the most widely used; (and) on the other hand, the gyrocompass, which is based on the rapid rotation of the free gyroscope with almost zero error. There are also gyromagnetic compasses, which combine the magnetic with the gyroscopic compass, yet they are very rare and of limited specialized use.

The compasses in ships are used for several purposes. Their main purpose is to indicate the course of the ship to the helmsman and the navigation officer. These compasses are called directing ones and are located at a point where the helmsman can see their indications clearly. Another important use is the possibility they give to the navigation officer to count with accuracy the absolute bearing of auspicious land points as well as of other ships in relation to the north. These compasses are called diopter compasses and are usually found on the wings of the open bridge, on the right and on the left, allowing the navigator to attain a true bearing of apparent land points or of other ships, in other words to count the angle of objects from the magnetic north. The «absolute bearing» is of particular value, since it is plotted on the map more accurately than the «relative bearing», which measures the angle formed by the observed object with the course of the observer’s ship and which changes as the observer’s ship turns. Finally, there are also smaller compasses for use in dinghies such as the portable ones, which provide very little accuracy.