trace element

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A chemical element required in minute quantities by an organism to maintain proper physical functioning.
2. n. A minute quantity or amount: "The trace elements of belief vanish when it becomes apparent that the . . . officer . . . never has suffered the indignity of combat” ( Lewis H. Lapham).

Wiktionary

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

WordNet 3.0

1. n. an element that occurs at very small quantities in the body but is nonetheless important for many biological processes