Not a form, what you get when you sit in front of an old guy, just a fan term.
Vegeta: WHAT WAS THAT? MYSTIC? Mystic's not a form!! It's just a fan term! OH GEE, you just sit in front of some old guy for a day and all of sudden, OW, you're Mystic! You didn't even transform, you just stood there and said 'Mystic'! And not to mention, you don't even belong in this era Gohan!! Adult Gohan doesn't belong in the Cell Era! Y-You know something Gohan...you're a loser.....J-Jus-Just go home, we don't need you!