Proline (abbreviated as Pro or P) is an a-amino acid, one of the twenty DNA-encoded amino acids. It is not an essential amino acid, which means that humans can synthesize it.
References
Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISNB 1-57259-153-6.