The angular relationship between two planets or a planet and an angle. Zodiacal aspects are based upon zodiacal longitude; parallels and counter parallels are based on declination, the position of points of interest relative to the celestial equator. Zodiacal aspects are also called harmonics.
The angular separation between two bodies, measured along a great circle, is termed Aspect. Each one of a set of specific number of degrees has an Astrological name and symbol and describes a way in which the energies of the two bodies influence each other. In additions, certain groups of aspects have specific names (Stellium, Grand Trine, Grand Cross, T-Square, Yod, etc.). Whole books have been written on this subject alone. See Planets in Aspect. Also see Applying