Entry 1 sense Webster, 1913 fibroin /(fī"brō̇*ĭn [or] fĭb"rō̇*ĭn)/ · fi·bro·in · IPA 01 n. A variety of gelatin; the chief ingredient of raw silk, extracted as a white amorphous mass. 1. A variety of gelatin; the chief ingredient of raw silk, extracted as a white amorphous mass.(Chem.)