amino acid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An organic compound containing an amino group (NH₂), a carboxylic acid group (COOH), and any of various side groups, especially any of the twenty compounds that have the basic formula NH₂CHRCOOH, and that link together by peptide bonds to form proteins or that function as chemical messengers and as intermediates in metabolism.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun organic chemistry Any organic compound containing both an amino and a carboxylic acid functional group.
• noun biochemistry 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun organic compounds containing an amino group and a carboxylic acid group