How your body is angled in the water — the goal is horizontal, with shoulders, hips and knees in one line. Buoyancy controls your depth, trim controls your attitude.
Buoyancy and trim get mixed up constantly: buoyancy decides whether you rise or sink, trim decides the angle you hold while doing it. You can get each one wrong independently — the classic case is a diver hovering perfectly neutral but tilted head-up. That angle drags like a half-open umbrella, and every kick pushes water downwards onto sediment or coral. The fix rarely lies in more effort but in distribution: move the tank higher or lower, add trim pockets to the tank band, secure hoses and consoles tight against your body. Good trim holds itself — if you have to hold it actively, the problem is your setup, not your abs. One self-test that never lies: stop moving completely and watch which way your body tips.