In thermodynamics, internal work, symbol J, or “interior work”, refers to the movement of the constituent atoms and molecules of a body, under the influence of a force, specifically the force which the molecules of a body exert among themselves, dependent on the nature of the body.  In short, internal work is the work the molecules of a body do on each other. It is a energy quantification internal to the system.
1. Clausius, Rudolf. (1879). The Mechanical Theory of Heat (pg. 27). London: Macmillan & Co.