What is thermal energy defined as?

Thermal energy is defined as the internal energy associated with the random motions of the particles within a substance. It is directly related to the temperature of the substance; as the temperature increases, the kinetic energy of the particles also increases, contributing to the overall thermal energy. Additionally, thermal energy plays a crucial role in phase changes, such as melting and boiling, where energy is absorbed or released, causing a change in state without necessarily changing temperature.

Thus, option C accurately encompasses both the relationship of thermal energy to temperature and the influence it has during phase changes. The other options do not effectively capture the comprehensive nature of thermal energy. For instance, potential energy in a substance does not account for the temperature or phase aspect, and energy associated with motion pertains more to kinetic energy, not the internal energy within a substance.