A medical term first coined in 1984 to describe the series of mental ailments suffered by a former Dungeon Master who has stepped down or been usurped by a newer, more liberal replacement.
The most common symptoms include nerdrage, an obsessive amount of referral to Dungeons and Dragons rulebooks, and the constant, repetitive, and overly false assurance by him that he/she is "definitely not fighting with anybody." The poor excuse "my campaign was a tutorial" is also commonly used by the former DM to try cover up how unoriginal, boring, and clichéd their adventure was.
Often, the sufferer of Post-DM Depression will temporarily have delusions of imaginary members of a "new, awesome group" that the new DM is "totally not invited to." These delusions normally fade quickly once the new DM states that he is happy to hear that the old DM was able to form such a great group so quickly, and hopes that they can still be friends.
Currently, the only known cure for Post-DM Depression is World of Warcraft, Guild Wars, or some other pointless online game where the former DM can become a guild leader and boss around torrents of pre-teens in a strict, structured environment that does not allow room for the creativity, imagination, and open-mindedness required to lead a party of campaigners in D&D.
Don't worry about it; He's just suffering from Post-DM Depression, because the entire party agrees that I am doing a better job as Dungeon Master than he ever could.
...I only understood, like, three words of that geeky gibberish.
You must have failed your Will check. There's an amulet for that, you know.