compass. 1 In mathematics, an instrument for making circles and measuring distances. Frequently called a pair of compasses, it consists of two metal legs with one end of each attached to a pivot to form a V-shaped device. The free ends are pointed; a pen or pencil may be substituted for one of the points. 2 In navigation, an instrument for determining direction. The mariner's compass consists of a magnetic needle freely suspended so that in the earth's magnetic field it turns until aligned with the magnetic north and south poles. Declination is the angle between the magnetic needle and the geographical meridian. Use of the compass by the early Chinese is probably legendary. The first known reference in European literature dates from the 12th cent. Another more accurate form of navigational compass is the gyrocompass. It consists essentially of a rapidly spinning, electrically driven rotor, suspended in such a way that its axis automatically points along the geographical meridian. The gyrocompass is unaffected by magnetic influences. This compass came into wide use in warships and aircraft during the Second World War. See gyroscope.