Here's the breakdown:

* Heat of fusion is the amount of energy required to change a substance from a solid to a liquid at its melting point.

* It does occur in a substance. It's a property of matter, specifically how much energy is needed to break the bonds holding the molecules in a solid structure and allow them to move freely as a liquid.

Example: The heat of fusion of water is 334 joules per gram. This means 334 joules of energy are needed to melt one gram of ice at 0°C into one gram of water at 0°C.

Let me know if you'd like more information about the concept of heat of fusion!