the GNU version of the Collaborative International Dictionary of English
n. An accessory plume arising from the posterior side of the stem of the contour feathers of many birds; -- called also aftershaft. See Illust. of feather.
The Century Dictionary and Cyclopedia
n. In ornithology, the aftershaft or hyporachis of a feather; the whole of the supplementary plume which springs from the stem of the main feather at the junction of calamus and rachis.