In the past 4 or so years, I know four different people who have had a "mental breakdown" but I'm a bit embarrassed to say I don't actually know really what a mental breakdown is.
They have all presented with their breakdown in completely different ways, are all recovering in completely different ways, and have all given me different descriptions of what their breakdown entailed, so it's left me a bit confused about why there's a catch all term for something that's obviously very complex, and what classes as a breakdown as opposed to depression or an anxiety attack.