The Collaborative International Dictionary of English v.0.48:

Plumule \Plu"mule\, n. [L. plumula, dim. of pluma a feather; cf.
   F. plumule.]
   1. (Bot.) The first bud, or gemmule, of a young plant; the
      bud, or growing point, of the embryo, above the
      cotyledons. See Illust. of Radicle. --Gray.
      [1913 Webster]

   2. (Zool.)
      (a) A down feather.
      (b) The aftershaft of a feather. See Illust. under
          Feather.
      (c) One of the featherlike scales of certain male
          butterflies.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plumula \Plu"mu*la\, n.; pl. L. Plumule, E.-las. [L. See
   Plumule.]
   1. (Bot.) A plumule.
      [1913 Webster]

   2. (Zool.) A down feather.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plumule
    n 1: down feather of young birds; persists in some adult birds