What is a Platonic Friendship?

I'd say it's a friendship with the platonic aspect of it highlighted for whoever you're talking to. Some people tend to see "only friendships" between people of opposite genders as impossible or hard to believe, so a person with such a friendship would naturally try to ensure the true nature of the relationship is not misunderstood and thus calls it a "platonic friendship".

I've never heard of a platonic friendship since platonic does mean friends only. I've heard of platonic relationships which is where you are just that: friends. There are no sexual or physical feelings whatsoever.

well a platonic would be a friendship where sexuality never enters the friendship, I don't see much of a difference between a platonic and regular friendship, if its an actually friendship it should always be platonic

This term arose awhile ago when the idea of a close friendship between a male and a female couldn't be viewed as just a friendship. Any close male female contact had to involve sex, but this wasn't true so the birth of the platonic relationship; a male-female relationship without sex.