Often shortened to SRB. Two reusable rockets on the space shuttle that use solid (actually kind of soupy/solid) fuel made of aluminum powder and ammonium perchlorate. Each is about 150 feet tall and together they provide, for about two minutes, three-quarters of the thrust needed to launch the orbiter and external tank into orbit. Once ignited, they cannot be shut off.
We had a baby guinea pig once who used to jump vertically with a little squeak when she was happy, which was quite often, so we named her Booster.