Heat is the opposite of cold. Simply heat is the sum of kinetic energy of atoms or molecules. In thermodynamics, heat means energy which is moved between two things when one of them is hotter than the other.
Adding heat to something increases its temperature, but heat is not the same as temperature. The temperature of an object is the measure of the average speed of the moving particles in it. The energy of the particles is called the internal energy. When an object is heated, its internal energy can increase to make the object hotter.
Heat can also be defined as the amount of thermal energy in a system. Thermal energy is the type of energy that a thing has because of its temperature.