Definition of "by-product"

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.

1911, “History of English Agriculture”, in 1911 Encyclopædia Britannica

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.

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