The truest depression you can have is when you feel down for a long period of time characterised by a total lack of enthusiasm and motivation for life, accompanied by feelings of helplessness and kind of like no future.
And about manic depression, its called Bipolar disorder now, where you are down for about a week to a month and then you suddenly have a sense of euphoria with racing thoughts and excessive energy and irritability. Then you go back to the depressive episode, and it just keeps happening in a cycle.