Protein (noun)

1. Any of a class of nitrogenous organic compounds that consist of large molecules composed of one or more long chains of amino acids and are an essential part of all living organisms, especially as structural components of body tissues such as muscle, hair, collagen, etc.
2. A substance that is essential for the growth and repair of body tissues.

Origin:

Late 19th century: from greek prōteios "primary", from prōtos "first".

Examples:

1. The human body needs a variety of proteins to function properly.
2. Egg whites are a good source of protein.
3. Protein is an essential part of a healthy diet.
4. The protein in milk is called casein.
5. Many bodybuilders consume protein supplements to help build muscle.