Bit of a vague question. Moreover, the notion of a decisive battle is a slightly contested one--everyone thinks they know what it is, but they can't agree on what exactly it is. That said, the most common interpretation of decisive military operations/battles/campaigns is one where at least the immediate enemy force is defeated to an extent sufficient to prevent it from remaining an effective presence in the field. As many generals have idealized it throughout history (particularly after Napoleon), a great battle causing the collapse of the enemy army as a fighting force (such as at Austerlitz or Jena-Auerstadt during the Napoleonic Wars, or Koniggratz during the Austro-Prussian War, or Metz and Sedan during the Franco-Prussian Wars, also what the Germans hoped the Schlieffen Plan would achieve). For a couple of decades prior to WW1, the battle of Cannae was held by many military theorists and planners (especially in Germany) as being the exemplar of a decisive engagement--innovative Carthaginian tactics leading to the near wholesale slaughter of the Roman force (what they ignore, of course, is that the battle was strategically irrelevant at best, it failed to achieve anything beyond a bloody field and continued years of security for an ineffective Carthaginian force in Italy. It had no political impact on Rome. At worst it was counterproductive, as it was a major impetus in impelling the Romans to fight in Spain and, later, Africa).

So now that we've approximately established a basic concept of decisiveness, indecisiveness would be the opposite. Therefore, campaigning which does not lead to major battles and battles which do not lead to the destruction of the enemy army. Clausewitz somewhat denigrates what he called the "cabinet wars" of the pre-Napoleonic era as being indecisive because it was the strategic fashion of the time to maneuver without bringing the enemy to battle. (Of course, battle is necessarily unpredictable in its results, whereas maneuver is somewhat more so.) So "indecisive campaigning" would probably feature a lot of maneuver, a lot of raiding and skirmishing (against the enemy forces, his logistics, as well as enemy civilians), but relatively little major bloodshed and probably not any that might have had a serious chance at definitively defeating the enemy army.

The only thing I would add to /u/kasirzin and /u/elos_ answers is that theres a more basic definition of decisive warfare. Literally, it is warfare which decides something. Kasirzin talks about Clausewitz-ian decisive warfare, while Elos talks about Napoleonic decisiveness. But there have been decisive conflicts both before and after both eras.

So while theyve given you a good specific definition, I would also point out that warfare which is indecisive is really war which doesnt settle anything. In your what-if question, laying siege or conquering some territory doesnt really automatically make warfare decisive. Take, for example, John Churchill's attack on the Non Plus Ultra lines during the war of Spanish Succession. Marlborough launched several successful sieges against French fortresses in Holland. However, due to political factors, the English were never able to capitalize on Marlborough's victories. Those sieges ultimately became indecisive because they were not properly exploited.

And that brings up the really tough philosophical problem with Decisive warfare. Napoleon, the greatest proponent of decisive warfare, ultimate lost. His many army shattering battles never permanently defeated his enemies, while Napoleon's decisive defeat at Leipzig didnt end his threat. So how decisive was Napoleon, and how decisive can one battle or campaign really be?