Definitions of zero:

• noun:   the quantity that registers a reading of zero on a scale
• noun:   a mathematical element that when added to another number yields the same number
• noun:   a quantity of no importance
• verb:   adjust (as by firing under test conditions) the zero of (a gun)

  Example: "He zeroed in his rifle at 200 yards"

• verb:   adjust (an instrument or device) to zero value
• adjective:   indicating the absence of any or all units under consideration

  Example: "A zero score"

• adjective:   indicating an initial point or origin
• adjective:   of or relating to the null set (a set with no members)