The term for the average speed of molecules in a substance is known as temperature. This quantity reflects the kinetic energy of molecules, as faster-moving molecules correspond with higher temperatures, while slower-moving molecules align with lower temperatures. It's an essential concept in both thermodynamics and kinetic theory, influencing the properties of materials, phase changes, and reaction rates. Temperature provides insight into various physical and chemical processes and plays a critical role in fields like meteorology, engineering, and biology, affecting everything from weather patterns to cellular metabolism.