Actually I think it's more of a Greek concept. Plato's idea of love almost sneers at heterosexual relationships and marriages as well as homosexual relationships that center on sex. While the term has gotten warped in out modern view, "Platonic love" is that which transcends friendship, romance, and (most importantly) sex. Ideally it was meant to occur between an older teacher or mentor figure and his student, but Plato was not above suggesting bisexuality as a realistic possibilty either. strauss_monster gives us the traditional Greek definitions; I'd suspect you want "philia," arguably the strongest and/or greatest love that a person can have or offer.

But back to the point. sistermagpie, I think you've got the right idea reaching into a historical context for socially "acceptable" homoeroticism, but I also believe you fall a little short in your view. Elizabethan England offers wonderful examples of more-than-friends male-male relationships, particularily in Shakespeare's early sonnets and his works Two Gentlemen of Verona and The Merchant of Venice. Obviously, unfathomably close heterosexual friendships that hold a degree of homoeroticism are much more common than people would lead you to believe but hardly can be called slashy; the homoeroticism goes unacknowledged and hence loses a major component necessary to slash fic, original or fandom-inspired. IMO, inserting homosexuality or acknowledged homoeroticism into a situation or relationship where it previously did not exist constitutes slash. If two (or more?) characters are homosexual or exploring that possibility in their canon and then homosexuality is introduced, that doesn't feel like slash so much as simply fic.