Definition:
The bottom bracket is the part of the bike that connects the crankset to the bicycle frame and allows the crankset (made up of pedals, crank arms and chain rings) to rotate freely. The crankset is either directly connected to the bottom bracket, or runs through it depending on the type, and bearings in the bottom bracket allow the a spindle to rotate when the cyclist turns the pedals.
The bottom bracket is located inside the bottom bracket shell, which is the lowest part of the bike frame and where the seat tube, down tube and chain stays all converge.
