The Collaborative International Dictionary of English v.0.48:

Stoichiometry \Stoi`chi*om"e*try\, n. [Gr. ? a first principle,
   or element + -metry.]
   The art or process of calculating the atomic proportions,
   combining weights, and other numerical relations of chemical
   elements and their compounds.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stoichiometry
    n 1: (chemistry) the relation between the quantities of
         substances that take part in a reaction or form a compound
         (typically a ratio of whole integers)