The Collaborative International Dictionary of English v.0.48:

Intrusive \In*tru"sive\, a.
   Apt to intrude; characterized by intrusion; entering without
   right or welcome.
   [1913 Webster]

   Intrusive rocks (Geol.), rocks which have been forced,
      while in a plastic or melted state, into the cavities or
      between the cracks or layers of other rocks. The term is
      sometimes used as equivalent to plutonic rocks. It is
      then contrasted with effusive or volcanic rocks. --
      In*tru"sive*ly, adv. -- In*tru"sive*ness, n.
      [1913 Webster]