The Collaborative International Dictionary of English v.0.48:

Haustorium \Haus*to"ri*um\ (h[add]s*t[=o]"r[i^]*[u^]m), n.; pl.
   Haustoria (h[add]s*t[=o]"r[i^]*[.a]). [LL., a well, fr. L.
   haurire, haustum, to drink.] (Bot.)
   One of the suckerlike rootlets of such plants as the dodder
   and ivy. --R. Brown.
   [1913 Webster]

WordNet (r) 3.0 (2006):

haustorium
    n 1: a root-like attachment in parasitic plants that penetrates
         and obtains food from the host