any disease of the mind; the psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention
a person who contributes to the fulfillment of a need or furtherance of an effort or purpose; "my invaluable assistant"; "they hired additional help to finish the work"
an adviser to the court on some matter of law who is not a party to the case; usually someone who wants to influence the outcome of a lawsuit involving matters of wide public interest