A bottom bracket is what attaches the crankset to the bike frame.
Tuftronic10000 via Flickr
Definition: A bottom bracket—which consists of a spindle and bearings—connects the crankset to the bike frame. The crankset is attached to the spindle. When a bike rider turns the pedals, the bearings are what allow the spindle and crankset to rotate freely. A short, wide tube in the bottom of the frame, called a bottom bracket shell, holds the bottom bracket inside.