The Collaborative International Dictionary of English v.0.48:

By-product \By"-prod`uct\, n.
   A secondary or additional product; something produced, as in
   the course of a manufacture, in addition to the principal
   product.
   [1913 Webster]

WordNet (r) 3.0 (2006):

by-product
    n 1: a secondary and sometimes unexpected consequence [syn: by-
         product, byproduct]
    2: a product made during the manufacture of something else [syn:
       by-product, byproduct, spin-off]