A nutrient is a chemical compound that an organism must take in from its environment to survive. Most of the nutrients are or derive from other living organisms.
Water and oxygen are chemicals necessary for life, but they are not usually regarded as "nutrients".
An essential nutrient is a nutrient required for normal body functioning that either cannot be synthesized by the body at all, or cannot be synthesized in amounts adequate for good health (e.g. niacin, choline), and thus must be obtained from a dietary source.