Among the four men's major championships in golf, the PGA Championship is unique in that there are no amateurs in the field. There are zero non-professionals competing in the second men's major of the calendar year.
But is it possible for an amateur to compete in the PGA Championship, or does a golfer have to be a professional?
The PGA of America runs the PGA Championship, and PGA stands for the Professional Golfers Association. This doesn't mean nothing to the PGA of America. They want only golfers who work in golf for a living -- professionally, even -- to compete for the PGA of America Championship and the Wanamaker Trophy. So, in order to be in the PGA Championship field, a golfer must be a professional.
Conceptually, an amateur could qualify for the PGA Championship were they to win one of the other three majors in the prior five years or somehow win The Players Championship, which doesn't invite amateurs to compete. The PGA of America wouldn't leave such an outstanding player out of the tournament.
However, at the very top of the PGA of America exemption criteria, the PGA of America states: "All contestants in the PGA Championship (except international players) must be a PGA of America member in good standing."
Obviously, an amateur golfer isn't going to be a member of the PGA of America in good standing. Otherwise, they would be a professional golfer. So, while it's possible for an amateur player to compete in the PGA Championship in an extraordinary, unprecedented circumstance, it is extremely unlikely.