The ascending node (Ω) is one of the orbital elements that must be specified in order to define the orientation of an elliptical orbit. The other orbital elements are inclination, and the argument of perihelion (ω).
The ascending node is usually quoted as the angular position at which a celestial body passes from the southern side of a reference plane to the northern side, hence ‘ascending’. For objects orbiting the Sun, the most convenient reference plane is the orbital plane of the Earth. In other systems, such as orbits of satellites around a planet, a more appropriate reference plane must be identified.
http://astronomy.swin.edu.au/cosmos/a/ascending+node