Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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