Orb
In plain words
How far off exact an angle is allowed to be and still count. Nothing in the sky lines up perfectly, so every angle is given a little room.
In the tradition's terms
The permitted deviation from exactitude, usually wider for the luminaries and narrower for the outer bodies.
In depth
There is no agreed table. Traditional practice orbs the *planets* rather than the aspect; modern practice orbs the aspect. The Oracle narrows the orb as a body slows, so a Saturn contact must be tighter than a Moon contact to be reported.