by-product

A by-product is a secondary or incidental product deriving from a manufacturing process, a chemical reaction or a biochemical pathway, and is not the primary product or service being produced. A by-product can be useful and marketable, or it can have severe ecological consequences.
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Noun
1. a secondary and sometimes unexpected consequence
(synonym) byproduct
(hypernym) consequence, effect, outcome, result, event, issue, upshot
(hyponym) epiphenomenon
2. a product made during the manufacture of something else
(synonym) byproduct, spin-off
(hypernym) product, production

(n.)
A secondary or additional product; something produced, as in the course of a manufacture, in addition to the principal product.
  

a secondary product produced during the process of manufacturing another.