From The School of Biomedical Sciences Wiki
The carboxyl group
(COOH on the right) is the C-terminus of an amino acid
All amino acids have the C-terminus also known as the carboxyl group. This end carries a negative charge and the carbonyl group present in the amino acid is a good hydrogen bond acceptor.
The C-terminus is an end point to the protein/amino acid, conversely the N-terminus is the starting point to the protein/amino acid.