In physics, heat is energy in a process of transfer between a system and its surroundings, other than as work or with the transfer of matter. When there is a suitable physical pathway, heat flows from a hotter body to a colder one. The pathway can be direct, as in conduction and radiation, or indirect, as in convective circulation.Because it refers to a process of transfer between two systems, the system of interest, and its surroundings considered as a system, heat is not a state or property of a single system. If heat transfer is slow and continuous, so that the temperature of the system of interest remains well defined, it can sometimes be described by a process function.Kinetic theory explains heat as a macroscopic manifestation of the motions and interactions of microscopic constituents such as molecules and photons.In calorimetry, sensible heat is defined with respect to a specific chosen state variable of the system, such as pressure or volume. Sensible heat transferred into or out of the system under study causes change of temperature while leaving the chosen state variable unchanged. Heat transfer that occurs with the system at constant temperature and that does change that particular state variable is called latent heat with respect to that variable. For infinitesimal changes, the total incremental heat transfer is then the sum of the latent and sensible heat increments. This is a basic paradigm for thermodynamics, and was important in the historical development of the subject.The quantity of energy transferred as heat is a scalar expressed in an energy unit such as the joule (J) (SI), with a sign that is customarily positive when a transfer adds to the energy of a system. It can be measured by calorimetry, or determined by calculations based on other quantities, relying on the first law of thermodynamics.