elective mutism

[ĭ-lĕktĭv]
n.
  1. A form of childhood mutism in which the ability to speak is intact, but there is a refusal to speak in almost all social situations.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.