The increase in length, area or volume due to the increase (in some cased decrease) in temperature. The stored energy in the molecular bonds between atoms changes when the heat transfer occurs. The length of the molecular bond increases as the stored energy increases.
Area thermal expansion - Area measures length times width and is two dimensional so it is squared. Expands twice as much as lengths do.
Linear thermal expansion - Linear measures distance or length and is one dimensional. Can only be measured in the solid state. The expansion is proportional to temperature change.
Volumetric thermal expansion - Volume measures length times width times height so it is three dimensional so it is cubed. Can be measured for all substances (liquid or solid) of condensed matter. Expands three times as much as lengths do.