Applying and separating
An applying aspect is still tightening: the faster body is closing on the exact angle. A separating aspect has peaked and is easing off. Same angle, different direction of travel.
Aspects are moments in a motion. Before two planets reach an exact angle, the aspect is applying: the faster of the two is closing the gap, and the pressure is building. After exactness it is separating: still in orb, still felt, but past its peak and unwinding.
The distinction is old and load-bearing. Horary astrologers built a craft on it, reading applying aspects as what is still to come and separating ones as what has already been decided. Transit work keeps the same instinct in a gentler form: a transit is a window that opens before the exact date and closes after it, with exactness as the crest.
The Moon, fastest of all, gives the cleanest demonstration. It applies to one aspect after another as it crosses each sign, and once it has none left to complete before leaving the sign, it runs void of course.