folie á deux

[fô-lē′ ä dœ, fŏl′ē]
n.
  1. A condition in which symptoms of a mental disorder occur simultaneously in two individuals who share a close relationship or association.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.