What is the latent heat of vaporization?

It is the additional amount of heat [measured in (the "big") calories ] required to cause boiling/vaporization of a liquid already at the boiling/vaporization temperature. Heat of vaporization is the amount of heat (540 calories/gram) needed to change a specific amount of liquid (1 gram) at it's boiling point into a gas without changing it's temperature. -Qwasas