Primary meaning: a secondary product obtained during the manufacture of something else. Used in industrial, economic, and scientific contexts.
Sulfur is obtained as a by-product of petroleum refining.
spin-off; outgrowth; side effect; consequence
Extended figurative use: an unintended but often beneficial result, a secondary effect, or a consequence of some action or development.
As a spin-off of this research, a new treatment method was discovered.
Traffic congestion worsened as a side effect of urban development.
This technology was born as a by-product of space development.
His success is a by-product of years of effort.
Harmful by-products can arise during the chemical reaction process.