The Collaborative International Dictionary of English v.0.48:

Spikenard \Spike"nard\, n.[For spiked nard; cf. G. spieknarde,
   NL. spica nardi. See Spike an ear, and Nard.]
   1. (Bot.) An aromatic plant. In the United States it is the
      Aralia racemosa, often called spignet, and used as a
      medicine. The spikenard of the ancients is the
      Nardostachys Jatamansi, a native of the Himalayan
      region. From its blackish roots a perfume for the hair is
      still prepared in India.
      [1913 Webster]

   2. A fragrant essential oil, as that from the Nardostachys
      Jatamansi.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spikenard
    n 1: an aromatic ointment used in antiquity [syn: nard,
         spikenard]

Easton's 1897 Bible Dictionary:

Spikenard
   (Heb. nerd), a much-valued perfume (Cant. 1:12; 4:13, 14). It
   was "very precious", i.e., very costly (Mark 14:3; John 12:3,5).
   It is the root of an Indian plant, the Nardostachys jatamansi,
   of the family of Valeriance, growing on the Himalaya mountains.
   It is distinguished by its having many hairy spikes shooting out
   from one root. It is called by the Arabs sunbul Hindi, "the
   Indian spike." In the New Testament this word is the rendering
   of the Greek nardos pistike. The margin of the Revised Version
   in these passages has "pistic nard," pistic being perhaps a
   local name. Some take it to mean genuine, and others liquid. The
   most probable opinion is that the word pistike designates the
   nard as genuine or faithfully prepared.