tight junction

[tīt]
n.
  1. An intercellular junction between epithelial cells in which the outer layers of the cell membranes fuse, reducing the ability of larger molecules and water to pass between the cells.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.