latent heat (symbol L) Heat absorbed or given out by a substance as it changes its phase at constant temperature. When ice melts, its temperature remains the same until it has been completely transformed into water; the heat necessary to do this is called the latent heat of fusion.

latent heat (lay-tĕnt) n. the quantity of heat absorbed or released when a substance changes state (e.g. from solid to liquid or from liquid to vapour) without any change in temperature.