Phase transitions is when a substance go through a change in its physical state, for example, solid to liquid, liquid to gas, etc. Energy in this case must be absorbed or released, and not necessarily it will be seen a change of temperature in this case. Mathematically, we calculate the energy released or absorbed by using the molar enthalpy of vaporization or molar enthalpy of fusion, as we can see in the following formulas:
ΔH = m*ΔHvap
ΔH = m*ΔHfusion
So phase transition is the energy required for a substance to change the physical state.