ze·ro 
(zîrō, zērō)
(zîrō, zērō)n. pl. ze·ros or ze·roes
1. The numerical symbol 0; a cipher.
2. Mathematics
a. The identity element for addition.
b. A cardinal number indicating the absence of any or all units under consideration.
c. An ordinal number indicating an initial point or origin.
d. An argument at which the value of a function vanishes.
3. The temperature indicated by the numeral 0 on a thermometer.
4. A sight setting that enables a firearm to shoot on target.
5. Informal One having no influence or importance; a nonentity: a manager who was a total zero.
6. The lowest point: His prospects were approaching zero.
7. Informal Nothing; nil: Today I accomplished zero.
adj.
1. Of, relating to, or being zero.
2.
a. Having no measurable or otherwise determinable value.
b. Informal Not any; no: "The town has ... practically no opportunities for amusement, zero culture" (Robert M. Adams).
3. Meteorology
a. Designating a ceiling not more than 16 meters (52 feet) high.
b. Limited in horizontal visibility to no more than 55 meters (180 feet).
4. Linguistics Of or relating to a morpheme that is expected by an established, regular paradigm but has no spoken or written form. Moose has a zero plural; that is, its plural is moose.
tr.v. ze·roed, ze·ro·ing, ze·roes
Phrasal Verbs: To adjust (an instrument or a device) to zero value.
zero in
1.
a. To aim or concentrate firepower on an exact target location.
b. To adjust the aim or sight of by repeated firings.
2. To converge intently; close in: The children zeroed in on the display of toys in the store window.
zero out
1. To reduce to zero.
2. To eliminate (a budget or budget item) by cutting off funding.
[Italian, from alteration of Medieval Latin zephirum, from Arabic ṣifr, nothing, cipher; see CIPHER.]