How is "thermal expansion" defined?

Thermal expansion refers to the propensity of matter to change its physical dimensions—such as shape, area, and volume—in reaction to changes in temperature. As a material is heated, its particles gain energy and occupy more space, leading to an increase in size. This principle is crucial in various applications, including engineering and construction, where materials expand and contract based on temperature fluctuations.

The other options either relate to different concepts or do not capture the full definition of thermal expansion. For instance, the first option focuses solely on temperature changes over time without considering the physical response of materials. The third option refers to a phase change rather than the dimensional changes of materials with temperature. The fourth option discusses contraction, which is a result of cooling, but does not encompass the overall behavior of materials during both heating and cooling processes. Thus, the statement that accurately presents the definition of thermal expansion is the one referring to the tendency of matter to change its shape, area, and volume in response to a change in temperature.