The state of hovering in the water without sinking or rising. You set it coarsely with the BCD and fine-tune it purely through lung volume.
Buoyancy is the single biggest lever on your air consumption. Constantly correcting up and down means pumping air into the BCD that you could have breathed instead. The fault usually starts before you even descend: too much lead. You then have to offset it permanently with air in the jacket, your centre of gravity slides towards your hips, and you hang upright in the water instead of flat. Once the lead is right, your lungs do the fine work — a slightly deeper breath lifts you, a longer exhale drops you, and the water takes a second or two to respond. What surprises most people is that buoyancy is hardest not at thirty metres but just below the surface, where air volume changes most in percentage terms. So practise it at three to five metres. And never control buoyancy by holding your breath — that is exactly the air that expands on the way up.