Definition: The building blocks of proteins. There are 20 different amino acids. They all have similar structures, with different side chains.
Of the 20 amino acids, 11 can be synthesized in the body. That leaves nine amino acids that you need to get directly from your diet, and they're called the essential amino acids.
Animal protein sources contain all the amino acids you need. Plant proteins contain most of the amino acids, but almost all plant-based foods are deficient in one or more essential amino acids.