Complex sentence: Equilibrium thermodynamics does, however, use the idealized concept of the "quasi-static process", which is a conceptual, though physically never actually possible, smooth mathematical passage along a continuous path of states of thermodynamic equilibrium.
Split into simpler sentences: Equilibrium thermodynamics does, however, use the idealized concept of the "quasi-static process". A quasi-static process is a conceptual (though physically impossible) smooth mathematical passage along a continuous path of states of thermodynamic equilibrium.