Zero

a quantity of no importance

a mathematical element that when added to another number yields the same number

the point on a scale from which positive or negative numerical quantities can be measured

the sight setting that will cause a projectile to hit the center of the target with no wind blowing

adjust (an instrument or device) to zero value

adjust (as by firing under test conditions) the zero of (a gun)

indicating the absence of any or all units under consideration

having no measurable or otherwise determinable value

indicating an initial point or origin

of or relating to the null set (a set with no members)