(1) One of a set of three coordinates, of a point, in a geocentric coordinate system. The coordinates are commonly represented by (λ, φ', r) where λ, the longitude, is the angle from a reference plane through the polar axis to the plane containing the polar axis and the point; λ', the geocentric latitude, is the angle from the equatorial plane to the radius vector to the point; and r, the geocentric distance or geocentric radius, is the distance of the point from the center. The letter Ψ is frequently used instead of φ'. Note that the origin (intersection of polar axis and equatorial plane, or centre of reference ellipsoid if one is used) need not be at the Earth's centre of mass or geometric center. The complete set of three geocentric coordinates is also called a geocentric position. (2) One of a set of coordinates designating the location of a point by means of (a) the angle from the plane of the celestial equator to a line from the centre of the Earth to the point and (b) the angle from the plane of a selected, initial geodetic meridian to that line. The first angle is called the geocentric latitude. The term geocentric longitude is not used for the second angle because that angle is the same as the geodetic longitude. This definition properly applies only to the exceptional case where the plane of the equator passes through the centre of the Earth and the polar axis of the reference ellipsoid coincides with the Earth's polar axis. (3) The longitude and latitude of a point (on the Earth's surface) relative to the centre of the Earth. The Earth's centre of mass is usually meant.