 After having looked at some simple work interactions, it is time for us to look at the thermodynamic definition of work. Before we do that, it is necessary for us to study what we mean by operational definitions. It is necessary to look at this because our thermodynamic definition of work will be a proper operational definition. The idea of operational definition was first proposed by Professor P. W. Bridgman, a well-known physicist, Nobel Laureate and a person who works significantly in thermodynamics and in the physics of high pressure. His books, The Nature of Physical Theory and The Nature of Thermodynamics are classics and those who are interested in the history of physics and the development of the formulations of physics should look up these books in an appropriate library. The Bridgman's idea of operational definitions which we will adopt is like this. An operational definition allows us to decide whether some item X is applicable by using a procedure and a procedure is defined as a series of operations which one can execute and these operations will finally tell us whether X is applicable or not, end up with yes, no. Let us consider the operational definition of a system. Now suppose we are given X, something, is X a system from the point of view of thermodynamics. What would be the operational definition? We will ask ourselves, question 1, does it define a region in space? Question 2, are all its boundaries properly defined in three? Hence, does it have a definite volume at any instant in time? Well, if the answer to all these question is yes, yes, yes, then yes, X is a system. This would be the operational definition of a system. Similarly, one can have operational definitions of a process, operational definition of a state, operational definition of many other things which we come across in physics. Thank you.