Cog

A toothed wheel which meshes with the bicycle chain in order to form gear ratios.
A cog is a metal piece mounted to the rear wheel hub of single-speed bicycles or to a cassette on multi-speed bicycles. Cogs are of varying sizes; more teeth make pedaling slower easier, which is useful when ascending slopes, and fewer teeth allow you to pedal faster on flat terrain. When gears shift, the chain travels from cog to cog to assist in altering the pedaling effort required. For extended rides, an effective cog configuration is necessary to maintain a consistent and strong pedaling cadence on various kinds of terrain. The majority of modern bicycles typically have between 8 and 13 cogs at the rear, providing a broad range of gears for training and competition.