Define the term "compression" in mechanical engineering.

The term "compression" in mechanical engineering refers to the reduction in volume that occurs when a material is subjected to an external compressive load. When forces are applied in such a way that they push inward on the material, the material is forced to occupy a smaller volume due to the increased pressure from these forces. This phenomenon is essential in understanding how materials behave under different types of loading conditions, particularly in structures and mechanical components where compressive forces are prevalent.

In contrast to some of the other options, which describe different mechanical behaviors, the concept of compression specifically focuses on the decrease in volume, not the increase in volume or elongation. For instance, the first option mentions an increase in volume due to heat, which refers to thermal expansion rather than compression. Similarly, the elongation of a material under tension represents tensile behavior, contrasting with the compressive nature of the correct definition. Lastly, while the option regarding the increase in stress due to external forces touches on stress analysis in materials, it does not accurately define compression itself. Thus, the definition of compression as the reduction in volume from an applied load is the most accurate in the context of mechanical engineering.