1) An emotionally and socially, but not sexually, involved relationship.
2) Nowadays, often a term for a friendship between a man and a woman which doesn't include a sexual element.
3) A friendship that lasts for as long as it takes for some
twats to
establish rumours about how the man wants to have off with the woman.
Platonic relationships often don't work because people become
hellbent on ending them, on the
assumption that they don't work, or because they are just jealous.