Definition of "professor"

Professor is what you become after teaching for twenty to thirty years.

2014 November 22, Michel Clasquin-Johnson, “What is the difference between a research professor and a professor”, in Quora‎

As for Religion, I have not said, much lesse meant irreverently of it, or of the Professors thereof.

1660, William Petty, Reflections upon some Persons and Things in Ireland, p. 170/1

This period in which Abraham the Jew lived was one in which Magic was almost universally believed in, and in which its Professors were held in honour;

1897, Samuel Liddell MacGregor Mathers (transl.) The Book of the Sacred Magic of Abramelin the Mage, Introduction, p. v

You could hear [...] pianos under the hands of whorehouse professors sounding like they came with keys between the keys.

2006, Thomas Pynchon, Against the Day, Vintage, published 2007, page 415