Noun

1. A quantity of no importance
2. A mathematical element that when added to another number yields the same number
3. The point on a scale from which positive or negative numerical quantities can be measured
4. The sight setting that will cause a projectile to hit the center of the target with no wind blowing

Adjective

1. Of or relating to the null set (a set with no members)

Verb

1. Adjust (an instrument or device) to zero value
2. Adjust (as by firing under test conditions) the zero of (a gun)