An aspect which is still in the process of forming, also referred to as an application.
It means that the faster of two planets (or other Sensitive Point such as the Ascendant or Medium Coeli) is approaching an exact aspect to the slower moving one.
The opposite is a Separating Aspect, an aspect which has passed the point at which it was exact. In this case the planets are moving away from aspect.
In most types of chart interpretation the applying aspect is considered to be stronger than the separating aspect because the energies are converging and the climax is still approaching. Consequently aspects formed by Transits, Directions, and Progressions are granted a larger Orb when in the applicative phase. The effects of a transit can even be observed several degrees before an exact aspect is formed, whereas after the transit planet has passed this point the effects may wane rather quickly.
In horary astrology a separating aspect indicates that the time for an event of interest or opportunity to occur has already passed. In deciding whether or not an event will happen in the future, the astrologer would consider only the applying or exact aspect as affirmative.