What defines a material that exhibits reversible changes in response to temperature variations?

A material that exhibits reversible changes in response to temperature variations is defined as a smart material. Smart materials are designed to respond dynamically to external stimuli, such as temperature, light, or electric fields. When temperatures change, these materials can adjust their properties or shape in a reversible manner, effectively "sensing" and reacting to the changes in their environment.

This ability to respond to temperature differences makes smart materials particularly valuable in various applications, from aerospace to robotics, where adaptability and resilience are crucial. Unlike traditional materials, which do not show such adaptive responses, smart materials offer innovative solutions where controlled modifications are essential.

Understanding this aspect distinguishes smart materials from other types of materials mentioned in the options, which may not exhibit the same dynamic responses to environmental changes.