The Collaborative International Dictionary of English v.0.48:

Owlet \Owl"et\, n. [Dim. of owl. Cf. Howlet.] (Zool.)
   A small owl; especially, the European species (Athene
   noctua), and the California flammulated owlet (Megascops
   flammeolus).
   [1913 Webster]

   Owlet moth (Zool.), any noctuid moth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

owlet
    n 1: young owl