trace element

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A chemical element present in tiny amounts.
• noun A chemical element required in minute quantities by an organism to maintain proper physical functioning.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry A chemical element present in a sample in very small quantities.
• noun biochemistry A chemical element in an organism’s diet that is required in very small amounts in order to maintain proper biochemical function; a micronutrient.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an element that occurs at very small quantities in the body but is nonetheless important for many biological processes