WordNet sense 1 (a mathematical element that when added to another number yields the same number):

cipher, nought, cypher

WordNet sense 2 (a quantity of no importance):

aught, naught, nil, nix, nothing, null, zilch, zip, nada, goose egg, zippo

WordNet sense 3 (the quantity that registers a reading of zero on a scale):

zero point

WordNet sense 4 (quantifier; used with either mass nouns or plural count nouns for indicating a complete or almost complete lack or zero quantity of):

no

WordNet sense 5 (being or denoting a numerical order in a series):

ordinal

WordNet sense 6 (being or denoting a numerical quantity but not order):

cardinal

WordNet sense 7 (adjust (as by firing under test conditions) the zero of (a gun)):

zero in