The Collaborative International Dictionary of English v.0.48:

Sea cucumber \Sea" cu"cum*ber\ (Zool.)
   Any large holothurian, especially one of those belonging to
   the genus Pentacta, or Cucumaria, as the common American
   and European species. (Pentacta frondosa).
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Trepang \Tre*pang"\, n. [Malay tr[imac]pang.] (Zool.)
   Any one of several species of large holothurians, some of
   which are dried and extensively used as food in China; --
   called also b[^e]che de mer, sea cucumber, and sea
   slug. [Written also tripang.]
   [1913 Webster]

   Note: The edible trepangs are mostly large species of
         Holothuria, especially Holothuria edulis. They are
         taken in vast quantities in the East Indies, where they
         are dried and smoked, and then shipped to China. They
         are used as an ingredient in certain kinds of soup.
         [1913 Webster]

WordNet (r) 3.0 (2006):

sea cucumber
    n 1: echinoderm having a flexible sausage-shaped body, tentacles
         surrounding the mouth and tube feet; free-living mud
         feeders [syn: sea cucumber, holothurian]