(1) Geographic meridian: Any vertical plane that contains geographic axis of earth is known as geographic meridian.
(2) Magnetic meridian: Any vertical plane that contains magnetic axis of earth is known as magnetic meridian. This is the vertical plane which passes through the axis of a freely suspended magnet.
(3) Angle of declination : The acute angle between magnetic meridian and geographical meridian at any place is called as angle of declination of that place.
(4) Angle of inclination or dip : It is defined as the angle made by the direction of magnetic field vector at any place with the horizontal. With reference to the angle of dip, the magnetic meridian at any place is also called the plane of dip circle.
(5) Geographic axis: It is the axis of rotation of the earth (also known as polar axis). This axis passes through the geographical poles of the earth.
(6) Magnetic axis: This is the line passing through earth’s magnetic north and south poles which is inclined at an angle of nearly 17 degrees with the geographic axis.
(7) Geographic equator: A great circle on the surface of the earth whose all points lie at equal distances from the geographic poles. It lies in a plane perpendicular to the geographic axis.
(8) Magnetic equator: A great circle lying in a plane perpendicular to the magnetic axis whose all points lie at equal distances from the magnetic poles.
(9) Horizontal component of earth’s magnetic field (H): It is the component of total intensity of earth’s magnetic field in the horizontal direction in magnetic meridian.