What is heat a measure of?

Heat is fundamentally a measure of energy, specifically the energy associated with the random motion of atoms and molecules within a substance. When heat is added to a material, its particles move more vigorously, which can lead to an increase in temperature. This energy transfer can occur through conduction, convection, or radiation. Understanding heat as a form of energy allows us to comprehend various physical processes, including changes in phase (like melting or boiling) and the transfer of energy between different systems.

The other options like pressure, mass, and velocity pertain to different scientific concepts that do not accurately define heat. Pressure relates to the force exerted by particles in motion against a surface, mass quantifies the amount of matter in an object, and velocity measures the speed of an object in a given direction, none of which encapsulate the nature of heat as energy.