In the heart, one of the two lower chambers. Blood arrives in the upper chambers (the atria), drops down into the ventricles, and then the ventricles squeeze hard and push it out — the right ventricle sends blood to the lungs, the left ventricle sends it to the whole rest of the body. That is why the left ventricle has the thickest muscle wall of the four chambers: it has the longest job. The word also names four fluid-filled spaces inside the brain, which hold and circulate the clear liquid that cushions it; you meet this use in medicine and brain scans. So 'ventricle' really means 'a hollow chamber inside an organ', and which organ depends on the context. It is a technical word — you meet it in biology class, in hospitals, and in health news about heart failure — not in ordinary chat. The adjective is ventricular: ventricular fibrillation, a dangerous heart rhythm that ambulance crews treat with a defibrillator.