The Collaborative International Dictionary of English v.0.48:

Umber \Um"ber\, n. [F. ombre ocherous ore of iron, terre
   d'ombre, It. terra d'ombra, literally, earth of shadow or
   shade, L. umbra shadow, shade. Cf. Umber, 3 & 4,
   Umbrage.]
   1. (Paint.) A brown or reddish pigment used in both oil and
      water colors, obtained from certain natural clays
      variously colored by the oxides of iron and manganese. It
      is commonly heated or burned before being used, and is
      then called burnt umber; when not heated, it is called
      raw umber. See Burnt umber, below.
      [1913 Webster]

   2. An umbrere. [Obs.]
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   3. [F. ombre, umbre, L. umbra.] (Zool.) See Grayling, 1.
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   4. [Cf. NL. scopus umbretta, F. ombrette; probably fr. L.
      umbra shade, in allusion to its dark brown color. See
      Umber a pigment.] (Zool.) An African wading bird
      (Scopus umbretta) allied to the storks and herons. It is
      dull dusky brown, and has a large occipital crest. Called
      also umbrette, umbre, and umber bird.
      [1913 Webster]

   Burnt umber (Paint.), a pigment made by burning raw umber,
      which is changed by this process from an olive brown to a
      bright reddish brown.

   Cologne umber, or German umber, a brown pigment obtained
      from lignite. See Cologne earth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

burnt umber
    n 1: dark brown pigment obtained by heating umber
    2: a medium brown to dark-brown color [syn: chocolate,
       coffee, deep brown, umber, burnt umber]