An orb is the allowed difference between a measured separation and the exact target angle of an aspect. If a square targets 90° and two points are 93° apart, their distance from exactness is 3°. They count as square only if the chosen policy allows at least that much.
Orb limits vary by astrological school, aspect type, and chart point. Wider orbs produce more aspect labels; narrower orbs emphasize closer geometry. There is no honest reason to hide the policy.
The separation from exactness can be calculated deterministically. Deciding that a conjunction or square remains symbolically meaningful within a particular tolerance is a convention.