amino acid

(plural: amino acids)

1. Any organic compound containing both an amino and a carboxylic acid functional group.

2. Any of the twenty naturally occurring α-amino acids (having the amino, and carboxylic acid groups on the same carbon atom, and a variety of side chains, that combine, via peptide bonds, to form proteins.

For definition 1: "Amino acids are organic compounds that contain both amino and carboxylic acid functional groups."

For definition 2: "These amino acids are the building blocks of protein."