A biosimilar product is a biologic product that is approved based on demonstrating that it is highly similar to an FDA‐approved biologic product, known as a reference product, and has no clinically meaningful differences in terms of safety and effectiveness from the reference product. Only minor differences in clinically inactive components are allowable in biosimilar products. 

Info gathered from: https://www.pfizerbiosimilars.com/characteristics-of-biosimilars