by-product; residual product
Technical term in chemistry for a substance produced incidentally during a reaction or process.
In this reaction, water is produced as a by-product.
副産物 is a broader term for any by-product, often used in economics or manufacturing, while 副生成物 is specifically a chemical by-product.
Compound of 副 (sub-, secondary) + 生成 (generation, production) + 物 (thing, substance).