Thermolabile describes substances that are easily destroyed, denatured, or altered by heat — from the Greek therme, heat, and the Latin labilis, liable to slip. Many enzymes are thermolabile, losing their activity when heated; several vitamins, notably vitamin C and thiamine, degrade with cooking; and some bacterial toxins and proteins fall apart at temperatures that leave others intact. The property matters throughout food science: pasteurisation is calibrated to kill pathogens while sparing as much as possible of what heat damages, blanching times balance enzyme inactivation against nutrient loss, and thermolabile vaccines and medicines need cold-chain protection. Knowing what heat destroys is as important as knowing what it kills.