Heat is an effect in thermodynamics. And heat is an energy transfer. So you can not just have heat in one system as the phenomenon includes that one thing gets warmer from the energy that coming from another thing. So speaking in terms of thermodynamics heat is different to the casual use of the word in which just one thing has the heat. And in order to be heat if energy is transferred between two systems then this may not happen due to the transfer of matter. And the symbol for the amount of transferred heat is usually Q or q. And this describes the energy being transferred.
So heat can be transfered in several ways. So conduction it is called when warm things touch each other. And then heat is transfered between those two. And if the objects do not touch but still there is an exchange in heat energy then the heat is transfered via heat rays. And that means that the warm objects send out a form of radiation. And that radiation is absorbed from the objects which leads to them becoming warmer. And the third way of inducing heat onto an object is friction. So when something slides along another thing then also it warms up. But friction also describes other ways of heat transfer. As it for example also stands for the heating up of objects due to gravitation. As for example the moons of the gas giants having heavy vulcanic activities because of the large planets tearing on them and deforming them in their strong gravitational field.
Even though heat usually flows from the warmer body to the colder body there are mechanical systems that turn this around. So a heat pump transfers the heat from the colder object to the warmer object. While a heat engine does put energy from the warmer to the colder object but it does so producing work. And then there is the technical device called heat spreader which also puts the warmth from a warmer object to a colder object in order to cool the first object.
And heat energy can be measured in joule. And as the ancient greek still thought of fire to be a form of matter this changed at some point. As James Clerk Maxwell in his Theory of Heat from 1871 defined: Heat cannot be treated as a material substance, because it may be transformed into something that is not a material substance, e.g., mechanical work. So heat is one of the forms of energy.
So many scientists contributet to our modern understanding of heat. As Joule for example is actually the name of a schientist. And he published a paper entitled The Mechanical Equivalent of Heat. And another person researching heat was Thompson who wrote Mechanical Theory of Heat in 1798. And yet another researcher on the topic of heat was Rudolf Clausius who is considered one of the central founders of thermodynamics.
