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

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "breakthrough":
   aggravated assault, aggression, amphibious attack, armed assault,
   assailing, assailment, assault, attack, avant-propos,
   banzai attack, blitz, blitzkrieg, boost, charge, coinage,
   counterattack, counteroffensive, coup de main, crippling attack,
   dead set at, descent on, discovery, diversion, diversionary attack,
   drive, exordium, flank attack, foreword, front matter,
   frontal attack, frontispiece, gas attack, head-on attack, hike,
   increase, infiltration, innovation, introduction, invention, leap,
   lightning attack, lightning war, mass attack, megadeath, mugging,
   neologism, new phase, novelty, offense, offensive, onset,
   onslaught, overkill, overture, panzer warfare, postulate, preamble,
   preface, prefix, prefixture, preliminary, prelude, premise,
   presupposition, proem, prolegomena, prolegomenon, prolepsis,
   prologue, protasis, push, run against, run at, rush, sally,
   shock tactics, sortie, strike, unprovoked assault, upgrade, verse,
   voluntary, wax