Buoyancy Control Device aka BCD. Various designs and styles, but basically a unit worn similar to a jacket that is an integral part of of one's Scuba Unit. Used primarily to control buoyancy at depth, and to offer positive buoyancy on the surface.

Bottom Time Bottom time is the physical time spent underwater from the moment one descends, until one starts ones

Buoyancy Positive buoyancy is where an object displaces water more than its own weight. It floats. Negative buoyancy is where an object displaces water less than its own weight. It sinks. Neutral buoyancy is where an object displaces water equal to its own weight. It hovers in the water like a helicopter hovers in the air. Or a diver hovers on a reef wall.