Why is a Frigate a Frigate (and not a Destroyer or a Corvette)

Forget the vague speculations about marginal differences in displacement, speed and armament. The real explanation of the difference can only be found in naval history. It amazes me that so few people, otherwise very knowledgeable about modern warships, know how the modern frigate-designation came to be. Most will give you vague generalizations, which won't leave you any wiser, probably more confused.