latent heat
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Put a thermometer in a pot of melting ice and you'll see something odd: you keep adding heat, but the temperature stubbornly stays at 0 °C until every last bit of ice is gone. Where did all that heat go? It went into the change itself, not into warming anything. That hidden, temperature-flat heat is the latent heat — 'latent' meaning hidden, because it doesn't show up on the thermometer.
Latent heat is the energy a substance absorbs or releases during a phase transition, while its temperature holds steady. The energy is spent breaking molecules free from one another (in melting or boiling) or letting them snap back together (in freezing or condensing). Each transition has its own value: the latent heat of fusion for melting and the latent heat of vaporization for boiling, the latter usually much larger.
Latent heat carries surprising punch. Steam scalds far worse than boiling water because, on touching your skin, it dumps its huge latent heat of condensation all at once. The same principle cools you when sweat evaporates, drives thunderstorms as water vapor condenses, and lets ice packs and refrigerants soak up heat. Releasing or absorbing energy at a constant temperature is exactly what makes phase changes such powerful energy reservoirs.
Melting one gram of ice needs 334 joules; boiling one gram of water away needs about 2,260 joules — nearly seven times more — which is why a kettle takes so much longer to boil dry than to reach 100 °C.
Vaporization soaks up far more latent heat than melting does.
Latent heat contrasts with sensible heat — the heat that does change temperature, the kind you feel on a thermometer. Heating ice from a freezer to steam involves both: sensible heat to warm each phase, latent heat to cross each transition.