The Collaborative International Dictionary of English v.0.48:

Graphite \Graph"ite\, n. [Gr. gra`fein to write: cf. F.
   graphite. See Graphic.] (Min.)
   Native carbon in hexagonal crystals, also foliated or
   granular massive, of black color and metallic luster, and so
   soft as to leave a trace on paper. It is used for pencils
   (improperly called lead pencils), for crucibles, and as a
   lubricator, etc. Often called plumbago or black lead.
   [1913 Webster]

   Graphite battery (Elec.), a voltaic battery consisting of
      zinc and carbon in sulphuric acid, or other exciting
      liquid.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plumbago \Plum*ba"go\, n. [L., from plumbum lead.]
   1. (Min.) Same as Graphite.
      [1913 Webster]

   2. (Bot.) A genus of herbaceous plants with pretty
      salver-shaped corollas, usually blue or violet; leadwort.
      [1913 Webster] Plumbean

WordNet (r) 3.0 (2006):

plumbago
    n 1: used as a lubricant and as a moderator in nuclear reactors
         [syn: graphite, black lead, plumbago]
    2: any plumbaginaceous plant of the genus Plumbago