protein means: You can find protein in animal or vegetable sources. It provides energy and performs many other functions. (in Cooking Dictionary)

What else does protein mean?

A group of organic macromolecules which contain complex combinations of carbon, hydrogen and oxygen. They also include nitrogen and often sulfur. The fundamental component of every living cell is protein. It includes many substances such as hormones and enzymes that help to maintain a healthy organism. Proteins are necessary for tissue growth and repair and can be found in foods like meat, fish and eggs as well as milk and other dairy products. (in Merlin Dictionary)