Buoyancy

The upward force water exerts on an immersed diver or object; divers speak of being positively, negatively, or neutrally buoyant.

Buoyancy is the upward push water gives anything immersed in it, and it defines a diver's state at every moment. Positively buoyant means you float upward, useful at the surface. Negatively buoyant means you sink, useful for the first meters of descent. Neutrally buoyant means the upward force exactly matches your weight, so you hover, and this is the target state for essentially the entire dive.

A diver's buoyancy is a live balance among fixed and changing factors: body composition, lead carried, tank material and how much gas remains in it, wetsuit compression with depth, air in the BCD or drysuit, and even lung volume with each breath. Descend and your suit compresses, so you add air to the BCD; ascend and that air expands, so you vent it. An aluminum tank growing lighter through the dive quietly shifts the balance too.

Buoyancy is the physics; buoyancy control is the skill of managing it, and it is the single strongest marker of a polished diver. Tracking your lead, suit, and tank in your logbook shortens the path to getting the balance right anywhere.