The Linus Pauling Institute likes to use the word micronutrient. This is not only because many world-renown authorities on micronutrients work at the LPI, but also because we maintain a database of scientifically accurate information regarding the roles of micronutrients in human health and disease, the Micronutrient Information Center (MIC).
Exactly what is a ‘micronutrient,’ anyway?
First, a strict definition: micronutrients are compounds that are absolutely essential to growth or maintenance of an organism (‘nutrient’) but may only be needed in small quantities (‘micro’).
In other words, they are small molecules that you absolutely need from your diet or you will get very ill and may risk death. You may only need small amounts of them at times, but that emphasizes how important they actually are. This is in contrast to macronutrients like protein and fats, which are needed in large quantities. 
Practically, we use the word ‘micronutrient’ as a general term. It encompasses the two main categories of compounds found in foods: vitamins and nutritionally essential minerals.
Vitamins are a group of organic (meaning they are derived from living things) compounds that are vital for normal functions in cells and the body. Without adequate levels of vitamins in our bodies, cells and tissues work inefficiently, biological processes begin to slow or stop, and deficiency diseases begin to appear.
Unfortunately, the term vitamin can describe a diverse set of compounds. It is difficult to sort them into categories because one vitamin may serve many roles, although some functions are unique to a particular vitamin that makes it important. So, rather than try to describe them, we feel it is better for you to read about each vitamin individually (as written on the MIC) to understand the biological functions.
Although most vitamins are required in the diet, there is one exception – vitamin D can be synthesized by the skin with energy from sunlight. Since our bodies can produce it, vitamin D is not truly a “vitamin”, but for people who are not exposed to the sun regularly it is a necessary dietary factor.
Minerals are a set of inorganic (meaning they are derived from the earth) compounds that are also found in living things. ‘Nutritionally essential minerals’ is a term that we use for those minerals that are needed to maintain normal biological functions.
Like vitamins, minerals have a diverse array of activities. Often minerals join with proteins to influence enzyme activity, but they also have several other functions, e.g., energy currency in cells, structural features in bone, and components of thyroid hormones.
There are other nutrients and essential parts of the diet that don’t fall under these two categories (e.g., choline, essential fatty acids), and there has been some debate on how to classify these compounds. However, like the vitamins and nutritionally essential minerals, these compounds are included in the MIC if you wish to know more about them and how they work in the body.
It is also worth noting that there a lot of compounds that are not classified as micronutrients, even though people will consume them in foods or as a supplement. Just remember that most micronutrients (with the exception of vitamin D) are essential parts of the diet needed in small amounts, so everything else falls under a different category.
