Orb
How far an aspect may sit from the exact angle and still count.
There's no agreed standard. Schools use anything from 1° to 10°, usually allowing more for the major aspects and less for minor ones; some assign orbs per planet instead, giving the Sun and Moon more room.
Orb isn't a detail — it sets the signal-to-noise ratio of the whole method. Roughly: ten natal points times five aspects gives fifty targets around the 360° circle; at a 3° orb each target covers 6°, so an arbitrary longitude has about an 83% chance of hitting one. Loosen the orb a little and everything "touches you", which says nothing at all.
So when this site matches sky events against a natal chart it uses 2° for the majors and 1.5° for sextiles — tighter than most transit work — and lists at most the two closest. The cost is that most days read "no close contact with your chart". That sentence is itself useful information, and it's what makes the days that do light up mean something.