Definitions of by-product on the Web:

• a secondary and sometimes unexpected consequence
• a product made during the manufacture of something else
  wordnetweb.princeton.edu/perl/webwn

• A by-product is a secondary or incidental product deriving from a manufacturing process, a chemical reaction or a biochemical pathway, and is not ...
  en.wikipedia.org/wiki/By-product

• A secondary product; something made incidentally during the production of something else; A side effect
  en.wiktionary.org/wiki/by-product

• Material, other than the principal product, generated as a consequence of an industrial process.
  www.nsc.org/ehc/glossary.html

• Material of some economic value produced in a process which is focused on extracting another material. For example palladium is produced as a by-product of platinum mining in South Africa.
  www.thebulliondesk.com/html/glossary.htm

• A secondary metal or mineral product recovered in the milling process such as copper and silver.
  www.goldcorp.com/investors/glossary/

• means a substance, listed in Schedule 1, which is incidentally manufactured, processed or otherwise used at the facility at any concentration, and released on site to the environment or disposed of.
  www.ec.gc.ca/pdb/npri/npri_glossary_e.cfm

• Retained catch which is not the primary target of a fishing operation.
  www.fish.wa.gov.au/glossary/GlossaryPage05.php

• A useful and marketable product or service that is not the primary product or service being produced. See also co-product.
  www.gdrc.org/uem/ait-terms.html

• A substance obtained incidentally during the manufacture or production of some other substance.
  www.conocophillips.com/newsroom/other_resources/energyglossary/glossary_b.htm

• A product of a chemical reaction other than the intended or desired one(s).
  www.chemeddl.org/collections/ptl/PTL/glossary/b.html

• A secondary or additional product; for example, the more common byproducts of coal ovens are gas, tar, benzol, and ammonium sulfate.
  www.greatmining.com/glossary-b.html

• The product generated in the process of producing the main product eg groundnut chunks are generated in the process of producing groundnut oil
  basiccollegeaccounting.com/frequently-used-terms-in-process-costing/

• A material created incidental to a production process, which can be sold for value.
  accounting-financial-tax.com/2008/09/inventory-terms-and-definitions-from-a-to-z/