barb; ramus (of a feather)
Ornithological term for the slender branches extending from the rachis of a feather.
Under a microscope, the barbs of a bird's feather show fine branching.
pinnule (of a feather)
Zoological term for the secondary branches on a feather barb; less common than the general 'barb' sense.
The even finer branches of a barb are called barbules (小羽枝).