Clark's level

[klärks]
n.
  1. Any of four levels that mark the invasion of malignant melanoma through the skin layers to the subcutaneous fat layer, each successive level indicating a worsening prognosis.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.