An aspect can be approaching its exact angle or moving away from it. Western astrologers call these two directions applying and separating.
Applying: nearing the exact angle
An aspect is applying when the gap from its exact angle is getting smaller. This describes changing positions around the chart, not planets getting closer together in space.
Separating: moving away from that angle
An aspect is separating when the positions are moving away from its exact angle. These two labels tell you the direction of change; neither means good or bad.
The angle gap alone does not give the direction
Two pairs can be the same distance from an exact aspect, while one is approaching it and the other is moving away. To tell them apart, check the movement as well as the gap.
A few common questions
Does an applying aspect mean something must happen?
No. It describes a direction of movement. The label alone cannot promise an event or tell you its outcome.
Sources & further reading
Astrology sources describe their own traditions and methods; they do not establish predictive accuracy. The writing exercises are original Lodestar editorial material.