phase change
A phase change is when matter switches between its states: solid, liquid, and gas. Ice melting into water, water boiling into steam, steam condensing back into droplets on a cold window, all are phase changes, the everyday transformations of stuff from one form into another.
Each phase change has a name and a partner: melting and its reverse freezing (solid to liquid); vaporization or boiling and its reverse condensation (liquid to gas); and sublimation and its reverse deposition (solid to gas directly, as when dry ice smokes). A phase change happens at a characteristic temperature for a given pressure, and it absorbs or releases latent heat while the temperature stays constant throughout the transition.
The constant temperature during a phase change is the key idea beginners should hold onto: while ice is melting, all the added energy goes into loosening the rigid structure into a liquid, not into making the mixture hotter, so the thermometer sits still at the melting point until the last of the ice is gone. Phase-change temperatures depend on pressure, which is why water boils at a lower temperature high up a mountain.
On a mountain 3000 m high, the thinner air lowers the pressure enough that water boils at about 90 C instead of 100 C, so pasta cooks more slowly up there.
Boiling point depends on pressure: lower pressure high up means water boils below 100 C.
Temperature stays constant during a phase change while latent heat is absorbed or released, and the transition temperature itself shifts with pressure.