The Collaborative International Dictionary
Radiolus
Radioli \Ra*di"o*li\ (r[.a]*d[imac]"[-o]*l[imac]), n. pl.; sing. Radiolus (r[.a]*d[imac]"[-o]*l[u^]s). [NL., dim. of L. radius radius: cf. L. radiolus a feeble sunbeam.] (Zo["o]l.) The barbs of the radii of a feather; barbules.
Wiktionary
radiolus
n. The barbule of a feather