A bicycle chain is a device that transmits power from the pedals to the rear wheel of a bicycle. It consists of a series of links that are connected together, with each link comprising two outer plates, two inner plates, and a roller. The roller sits between the inner plates and is what engages with the teeth on the chainring and cassette to transfer power.

Bicycle chains come in different widths, depending on the number of gears on the bike. Narrower chains are used for bikes with more gears, as they need to fit between the smaller gaps on the cassette. Chains also come in different lengths, which can be adjusted by removing links to fit the bike frame and drivetrain.