filling defect

n.
  1. A defect in the contour of part of the gastrointestinal tract, as seen by x-ray after contrast medium has been introduced, indicating the presence of a tumor or foreign body.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.