What is meant by the term "thermal expansion"?

The term "thermal expansion" refers to the tendency of materials to change in shape or volume as their temperature increases. When a material is heated, its particles gain energy and move apart, causing an increase in the material's overall size. This phenomenon occurs in solids, liquids, and gases, though the degree of expansion can vary significantly among different materials. Understanding thermal expansion is crucial in engineering and construction, where materials must accommodate changes in temperature to prevent cracking, warping, or structural failure.

The other options do not accurately capture the essence of thermal expansion. The idea of contraction with heat pertains to materials that experience a decrease in size as they cool, but that is not thermal expansion. The ability of materials to conduct heat relates to thermal conductivity, which is a separate property, while the cooling process for solidification is not directly connected to the concept of expansion caused by heat. Thus, the definition that focuses on changing shape or volume with temperature changes is the most accurate representation of thermal expansion.