potassium-40

n.
  1. A naturally occurring beta-emitting radioactive isotope of potassium having a half-life of 1.3 billion years; it is the chief source of natural radioactivity of living tissue.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.