In chemistry, a product is a substance, such as an atom, ion, molecule, molecular entity, or chemical species that is formed during a chemical reaction. [1] A reactant, conversely, is what goes into or exists at the start of a reaction. This is diagrammed below:

Reactants → Products

References
1. (a) Product - IUPAC Goldbook
(b) Daintith, John. (2004). Oxford Dictionary of Chemistry. New York: Oxford University Press.