The lower portion of the drillstring, consisting of (from the bottom up in a vertical well) the bit, bit sub, a mud motor (in certain cases), stabilizers, drill collar, heavy-weight drillpipe, jarring [..]

The bottomhole assembly is the portion of the drilling assembly below the drill pipe. It can be as simple as bit and drill collars or it can be very complex and made up of multiple components.

the lower end of the drill string comprising the drill bit, drill collars, heavyweight drill pipe and ancillary equipment.