Orb
The allowance around an exact aspect. A square is exact at ninety degrees; with a six-degree orb, anything from 84 to 96 counts. The tighter the orb, the stronger the aspect.
Aspects are defined by exact angles, but the sky rarely obliges to the arcminute, so every aspect is granted an orb: a margin within which it still counts. Orbs are a convention, not a law of nature, and practitioners set them differently, commonly six to ten degrees for major natal aspects, wider for the Sun and Moon, tighter for transits.
What nobody disputes is the gradient. An aspect half a degree from exact is a different experience from the same aspect at seven degrees, and the closest aspects in a chart deserve to be read first. That is why every aspect list on this site is sorted tightest first, with the orb printed beside it.
Whether the aspect is applying, still tightening toward exact, or separating, already past it, is the other half of the story.