Solidification or freezing is the term used for the process in which a liquid becomes a solid. Freezing is an exothermic process that also is an example of a phase transition.
This process occurs when the temperature of the substance (the liquid) is lower than its freezing point.
The three states, or phases, of matter are gas, liquid and solid. A phase transition happens when matter in one phase changes to another phase due to a change in either temperature or pressure.
Besides solidification, other transition phases include melting, vaporization and condensation. An example of the opposite of freezing is melting. With melting, a solid turns to a liquid.