- the length of the line segment joining a fixed point or origin to a given point.
- Astronomy.
- the straight line joining two bodies in relative orbital motion, as the line from the sun to a planet at any point in its orbit.
- the distance between two such bodies at any point in the orbit.
- a line joining a point in space to the origin of polar or spherical coordinates
- an imaginary line joining a satellite to the planet or star around which it is orbiting
