noun (n)

• used as a lubricant and as a moderator in nuclear reactors(noun.substance)

  Synonym:
  black lead, plumbago
  

  source: wordnet30
• 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.(noun)

  source: webster1913