Arthur Samuel (1959). Machine Learning: Field of study that gives computers the ability to learn without being explicitly programmed.
Came across this while doing some general work. I enjoy the history of AI, so many things that we take for granted now were unknown only 50 years ago. There are more modern (and more formal) definitions, but this one has an elegance to it.
Checkers is now solved (thanks Chinook), but that does not make the original 1959 results any less cool. As a matter of fact, I think that it might make the original results even more cool.