The bottom bracket is more than just the part of the frame where the down tube, seat tube and chain stays meet. The bottom bracket is the part that fits inside that part of the frame, and includes bearings and races, the axle assembly and the attachment mechanism for the crank arms. I know Sheldon uses the same term for both, but if you order a bottom bracket from a bike shop I guarantee you won't get a lug or casting. You'll get the innards.