Obliquity is the angle made between the plane of the ecliptic and the plane of the celestial equator.
This value is currently around 23 degrees 26 minutes and varies slowly over the centuries.
There are two methods in use.
Oriental/occidental to the Sun (Commonly used)
Mars, Jupiter and Saturn are oriental if they rise before the Sun, otherwise they are occidental.
The Moon, Mercury and Venus are oriental if they rise after the Sun, otherwise they are occidental.
Using whole sign houses, any planets in houses 10,11,12 or 4,5,6 are oriental, otherwise they are occidental.
A term used to describe a planet's position north and south of a selected reference plane.
Examples from the 'Wheel / Alternate Co-ordinate Systems' module.