It's been a while, but as a former teacher I could not leave it at that forever.

My definition refers to the economy itself. Your professor was apparently referring to the study of the economy. It is a given in 'mainstream' economics that even where there is enough there can be scarcity, in the sense that people have infinite wants.

I would agree that economists have done humanity a disservice by taking infinite desires to be a 'rational' starting point for the structure and (sanctioned) functioning of an economic system. Even beyond legitimating the pursuit of infinite desires as an approach to life, which is bad enough, they have made such an approach to life the very definition of rationality. It is anything but rational. Making that the starting point for an economic system is a recipe for environmental, social, and personal disaster on a planetary scale.

(Keeping in mind that my approach is not socialism, much less Marxism) to pretend that the existing system is somehow sacrosanct, that it is something other than the product of conscious actions of human beings seeking the infinite accumulation of wealth for some, all others be damned, is to substitute ideological dogma for rationality.