Buoyant or buoyancy is relative to the sinking or floating characteristics of an object in liquids. We can say objects are positively buoyant or have a positive buoyancy when they float. Inversely are negatively buoyant when they sink, and neutrally buoyant when they neither float or sink and remains at the same depth.
A wing is an air cell used for buoyancy control. It is normally composed of a bladder protected by a nylon outer shell. A wing can have a horseshoe shape or a donut shape. It is attached to the plate of a diving harness. Some brands of scuba gear started to offer one piece wing style or hybrid jacket to the main market.
The trim is the position of a diver relative to the surface. Having a good trim means that the diver can float in a horizontal position parallel to the surface. It is important as it reduce drag and increase overall efficiency of movement underwater.