The Collaborative International Dictionary of English v.0.48:

Labellum \La*bel"lum\ (l[.a]*b[e^]l"l[u^]m), n.; pl. L.
   Labella (l[.a]*b[e^]l"l[.a]), E. Labellums
   (l[.a]*b[e^]l"l[u^]mz). [L., dim. of labrum lip.]
   1. (Bot.) The lower or apparently anterior petal of an
      orchidaceous flower, often of a very curious shape.
      [1913 Webster]

   2. (Zool.) A small appendage beneath the upper lip or labrum
      of certain insects.
      [1913 Webster]