The Collaborative International Dictionary of English v.0.48:

bourtree \bourtree\ n.
   common black-fruited shrub or small tree (Sambucus nigra)
   of Europe and Asia; -- the fruit is used for wines and
   jellies.

   Syn: black elder, common elder, elderberry, European elder.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Elder \El"der\, n. [OE. ellern, eller, AS. ellen, cf. LG.
   elloorn; perh. akin to OHG. holantar, holuntar, G. holunder;
   or perh. to E. alder, n.] (Bot.)
   A genus of shrubs (Sambucus) having broad umbels of white
   flowers, and small black or red berries.
   [1913 Webster]

   Note: The common North American species is Sambucus
         Canadensis; the common European species (S. nigra)
         forms a small tree. The red-berried elder is S.
         pubens. The berries are diaphoretic and aperient. The
         European elder (Sambucus nigra) is also called the
         elderberry, bourtree, Old World elder, black
         elder, and common elder.
         [1913 Webster + WordNet 1.5]

   Box elder. See under 1st Box.

   Dwarf elder. See Danewort.

   Elder tree. (Bot.) Same as Elder. --Shak.

   Marsh elder, the cranberry tree Viburnum Opulus).
      [1913 Webster]

WordNet (r) 3.0 (2006):

bourtree
    n 1: a common shrub with black fruit or a small tree of Europe
         and Asia; fruit used for wines and jellies [syn:
         bourtree, black elder, common elder, elderberry,
         European elder, Sambucus nigra]