Or at least that’s one good definition. Here’s a few others that come to mind:
Depression is an unblockable spammer that gets your email address and starts sending messages to all the darkest corners of your inbox.
Depression is the feeling that no one’s ever loved you who didn’t feel obligated to do so.
Depression is the inability to believe that the future will be any different from the present, and that the present is inescapably bound by the past. It is at once a hopelessness born out of the idea that one is helpless, and a frustration at oneself because one feels so helpless, as if the feeling was something one chose to feel.
Depression is what turns “today” into “tomorrow” into “three days from now” and still leaves you feeling mentally exhausted when you finally accomplish whatever simple thing it was you meant to have done days ago.
See also: depression can fucking go to hell.