Drucker’s definition of Free Enterprise is pretty straightforward. It has four elements:
Governments set the rules of the economy but usually don’t participate by running businesses
Governments can operate natural monopolies or operations needed for national security;
Productive Resources owned privately;
Corporations competing in Markets and Market forces determine business decisions.
(Second one, about natural monopolies, seems less elegant than the others as it seems a pragmatic statement that acknowledges the state of affairs rather than states a basic principle of the economy.)