from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A condition in which symptoms of a mental disorder, such as the same delusional beliefs or ideas, occur simultaneously in two individuals who share a close relationship or association.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A rare syndrome in which a symptom of psychosis (particularly a paranoid or delusional belief) is transmitted from one person to another.
French : folie, madness + à, between + deux, two.(American Heritage® Dictionary of the English Language, Fourth Edition)
French folie à deux, "madness of two (people)". (Wiktionary)