1. A crystalline amino acid HOC6H4CH2CH(NH2)COOH obtained in its levorotatory L-form by the hydrolysis of proteins (as casein or fibroin) and also made synthetically in the racemic form; para-hydroxy/phenylalanine.
2. Either of two crystalline acids that are ortho and meta isomers of tyrosine.
ref: Webster's International Unabridged Dictionary