latent heat
Latent heat is the 'hidden' heat that a substance absorbs or releases while changing state (melting, boiling, freezing, or condensing) without its temperature changing at all. Put a thermometer in a glass of ice water and stir: it stays stubbornly at 0 C while the ice melts, even though heat is pouring in. That heat is going into the phase change, not into raising the temperature.
The energy needed to change the state of a mass m is Q = m L, where L is the latent heat, a property of the substance and the particular transition. The latent heat of fusion covers melting and freezing; the latent heat of vaporization covers boiling and condensing. During the change, the incoming energy goes into breaking the bonds that hold particles together rather than speeding them up, which is exactly why the temperature holds steady. For water, L_fusion ≈ 334 kJ/kg and L_vaporization ≈ 2260 kJ/kg.
Latent heat explains why sweating cools you (evaporating sweat carries away vaporization heat) and why a steam burn is far worse than a hot-water burn: when steam condenses on your skin it dumps its large latent heat of vaporization all at once, on top of the heat from cooling. The temperature staying constant during melting or boiling is the signature of latent heat at work.
Melting 1 kg of ice at 0 C into water at 0 C takes Q = m L = 1 x 334000 = 334000 J, the very same energy that, added to liquid water, would instead raise its temperature by about 80 C.
Melting ice absorbs a lot of energy at constant temperature; that energy is the latent heat of fusion.
Temperature does not change during a phase change: the latent heat goes into breaking or forming molecular bonds, not into speeding the particles up. A steam burn is severe precisely because condensing steam releases so much of it.