The Collaborative International Dictionary of English v.0.48:

bdellium \bdel"lium\, n. [L., fr. Gr. bde`llion; cf. Heb.
   b'dolakh bdellium (in sense 1).]
   1. An unidentified substance mentioned in the Bible (--Gen.
      ii. 12, and --Num. xi. 7), variously taken to be a gum, a
      precious stone, or pearls, or perhaps a kind of amber
      found in Arabia.
      [1913 Webster]

   2. A gum resin of reddish brown color, brought from India,
      Persia, and Africa.
      [1913 Webster]

   Note: Indian bdellium or false myrrh is an exudation from
         Balsamodendron Roxburghii. Other kinds are known as
         African bdellium, Sicilian bdellium, etc.
         [1913 Webster]

WordNet (r) 3.0 (2006):

bdellium
    n 1: aromatic gum resin; similar to myrrh

Easton's 1897 Bible Dictionary:

Bdellium
   occurs only in Gen. 2:12, where it designates a product of the
   land of Havilah; and in Num. 11:7, where the manna is likened to
   it in colour. It was probably an aromatic gum like balsam which
   exuded from a particular tree (Borassus flabelliformis) still
   found in Arabia, Media, and India. It bears a resemblance in
   colour to myrrh. Others think the word denotes "pearls," or some
   precious stone.