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

nought, cypher, zero

WordNet sense 2 (a quantity of no importance):

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

WordNet sense 3 (a person of no influence):

nobody, nonentity

WordNet sense 4 (a secret method of writing):

cryptograph, secret code

WordNet sense 5 (convert ordinary language into code):

code, encipher, encode, inscribe, encrypt, write in code

WordNet sense 6 (make a mathematical calculation or computation):

calculate, compute, figure, reckon, work out