Chemistry will tell you that an amino acid is any molecule that has both carboxylic acid and amino functional groups. They are the basic building units of a protein. In biochemistry, the shorter and more general term is used to refer to alpha amino acids. Those are amino acids wherein the amino and carboxylate functionalities are attached to a common carbon. Amino acids' residue is what's left of an amino acid once a water molecule has been lost in the formation of a peptide bond.