Proteinogenic amino acids are those 20 amino acids that are found in proteins and that are coded for in the standard genetic code. Proteinogenic literally means protein building.
Non-proteinogenic amino acids are either not found in proteins (like carnitine, GABA, or L-DOPA), or not coded for in the standard genetic code (like hydroxyproline). The latter often result from posttranslational modification of proteins.
