What invention, developed around 1765, was Watt's fundamental thermodynamic breakthrough?

Answer

The separate condenser.

The separate condenser allowed the used steam to be cooled and condensed in a separate chamber kept permanently cold, thus keeping the main working cylinder hot and drastically reducing fuel waste.

What invention, developed around 1765, was Watt's fundamental thermodynamic breakthrough?
HistoryinventorengineeringMechanicsteam engine