Plain English Meaning
A specific mental health condition that the FAA has decided makes a person ineligible to fly as a certificated pilot. If someone has one of these conditions, they cannot pass the medical exam required to fly.
Full Definition
A mental or emotional condition that, under FAA medical certification standards, prevents a person from being legally eligible to hold a pilot medical certificate. Examples listed in FAR Part 67 include established psychosis, bipolar disorder, severe personality disorder manifested by overt acts, and substance dependence or abuse.