Something we need in very small amounts to stay alive. The exact amounts of each vary from person to person, based on many factors (pregnancy, diarrhea, AIDS, age, gender, hydration levels, activity level, and weight to name a few).
The following are some examples of:
Dr. Rath Health Foundation. List of Micronutrients, 2016. http://www4.dr-rath-foundation.org/open_letters/img-nyt0506/micronutrients.htm. Accessed October 1, 2016.