A unit of weight equal to one millionth of a gram. Abbreviated "mcg" or "g." (The symbol is the Greek letter mu, commonly used for "micro.") Not to be confused with milligram, a unit 1000 times larger. Although invisible to the naked eye, a substance weighing just a few micrograms can be crucial to good nutrition. Examples: Vitamin A, Vitamin B6, Vitamin B12, and folate.