by-product

• byproduct
• bye-product (archaic)

From by- +‎ product.

• Homophone: biproduct

by-product (plural by-products)

1. Something made incidentally during the production of something else.
   • 1911, “History of English Agriculture”, in 1911 Encyclopædia Britannica:

     It was about this time that the first experiments were made (in Germany) with basic slag, a material which had hitherto been regarded as a worthless by-product of steel manufacture. A year or two later field trials were begun in England, with the final result that basic slag has become recognized as a valuable source of phosphorus for growing crops, and is now in constant demand for application to the soil as a fertilizer.

2. A consequence, especially a side effect.

   The many Indian restaurants all over Britain are a by-product of the large waves of immigration from the subcontinent.

   • 1961 February, R. K. Evans, “The role of research on British Railways”, in Trains Illustrated, page 92:

     One of the most obvious results of the B.R. Modernisation Plan has been the increasing use of diesel and electric traction; a less obvious by-product is the increase in track damage possible with the new forms of traction.

• waste product

• by-product on Wikipedia.Wikipedia