What did Joseph Black mean by the latent heat of fusion of water?

The British scientist Joseph Black discovered the concept of latent heat by investigating why substances do not undergo instantaneous phase changes when they reach their melting point or boiling point.1

Black observed that when ice reaches its freezing point, it does not melt suddenly; instead, it must absorb a significant quantity of heat to transition into a liquid state while remaining at a constant temperature. Through experiments comparing the warming rates of ice and water, he demonstrated that the heat absorbed by ice during this phase change—which he termed the latent heat of fusion—was substantial, equivalent to the amount of energy required to raise the temperature of the same mass of water by approximately 78 °C (140 °F).1

This insight revealed that heat can be absorbed by a substance without causing a change in its temperature, as the energy is instead utilized to overcome the forces holding the crystalline solid structure together.21