The Collaborative International Dictionary of English v.0.48:

breakthrough \break"through`\ n.
   1. a productive insight.

   Syn: discovery, find.
        [WordNet 1.5]

   2. a sudden change of situation, such as making an important
      discovery, that permits further rapid advances toward a
      goal.
      [WordNet 1.5 +PJC]

   3. (Mil.) a penetration of an enemy's defense in depth and
      strength.
      [WordNet 1.5] Breakup

WordNet (r) 3.0 (2006):

breakthrough
    n 1: a productive insight [syn: discovery, breakthrough,
         find]
    2: making an important discovery
    3: a penetration of a barrier such as an enemy's defense