The Collaborative International Dictionary of English v.0.48:

Plutonic \Plu*ton"ic\, a. [Cf. F. plutonique. See Pluto.]
   1. Of or pertaining to Pluto; Plutonian; hence, pertaining to
      the interior of the earth; subterranean.
      [1913 Webster]

   2. Of, pertaining to, or designating, the system of the
      Plutonists; igneous; as, the Plutonic theory.
      [1913 Webster]

   Plutonic action (Geol.), the influence of volcanic heat and
      other subterranean forces under pressure.

   Plutonic rocks (Geol.), granite, porphyry, and some other
      igneous rocks, supposed to have consolidated from a melted
      state at a great depth from the surface. Cf. Intrusive
      rocks, under Intrusive.

   Plutonic theory. (Geol.) See Plutonism.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plutonic
    adj 1: of igneous rock that has solidified beneath the earth's
           surface; granite or diorite or gabbro [syn: irruptive,
           plutonic]