Is the point where the Eastern Horizon intersects the Prime Vertical (a Great Circle passing through the Zenith and the Nadir of the Place) and the Celestial Equator (a Great Circle corresponding to the Earth's Equator projected onto the Celestial sphere.)
The East point is where the ascendant would be if a person were born on the equator, it represents how others see us, our symbol in public. Sometimes people tune into this point more that their ascendant, and this becomes the point activated in times of change, often it is aspected in times of death. I personally react to this point more than my ascendant in the natal chart, not the diurnal chart. People who tune into the EP more than the Ascendant are usually non reactive to the Arabic parts, which centres the calculations around the Ascendant.