Merriam-Webster Collegiate Dictionary - zero

 I. noun  (plural ~s; also ~es)  Etymology: French or Italian; French zero, from Italian ~, from Medieval Latin zephirum, from Arabic ?ifr  Date: 1598  1.  a. the arithmetical symbol 0 or O denoting the absence of all magnitude or quantity  b. additive identity; specifically the number between the set of all negative numbers and the set of all positive numbers  c. a value of an independent variable that makes a function equal to ~ +2 and -2 are ~s of f(x)=x2-4  2. — see number table  3.  a.  (1) the point of departure in reckoning; specifically the point from which the graduation of a scale (as of a thermometer) begins  (2) the temperature represented by the ~ mark on a thermometer  b. the setting or adjustment of the sights of a firearm that causes it to shoot to point of aim at a desired range  4. an insignificant person or thing ; nonentity  5.  a. a state of total absence or neutrality  b. the lowest point ; nadir  6. something arbitrarily or conveniently designated ~  II. adjective  Date: 1810  1.  a. of, relating to, or being a ~  b. having no magnitude or quantity ; not any ~ growth ~ tolerance  c.  (1) having no phonetic manifestation the ~ modification in the past tense of cut  (2) having no modified inflectional form a ~ plural  2.  a. of a cloud ceiling limiting vision to 50 feet (15 meters) or less  b. of horizontal visibility limited to 165 feet (50.3 meters) or less  III. verb  Date: 1913  transitive verb  1. to determine or adjust the ~ of (as a rifle)  2.  a. to concentrate firepower on the exact range of — usually used with in  b. to bring to bear on the exact range of a target — usually used with in  intransitive verb  1. to adjust fire on a specific target — usually used with in  2. to close in on or focus attention on an objective — usually used with in investigators are ~ing in on a suspect