The question arises because Crimean Peninsula is currently being occupied by thousands of well-armed men in uniforms that has no insignia who threaten to storm military installations and already occupied a few, as well as blocked many routes, etc. These people refuse to name the armed forces or nation. Everybody knows they are Russian forces, but Russia has officially disowned them: in response to the demand to call them off Putin declared that Russia has not deployed them and therefore cannot call them off.
Now, this raises a question: according to international law, are these people terrorists? Since they are not officially a part of any nation's armed forces, and yet they took control of land and population at the gunpoint (and fired a few shots already) how they are different from any other terrorist organization? I am not well-versed in it, but it seems to me that by the letter, disregarding the undeclared allegiance to Russia and considering only the official statements and their actions, they can indeed be declared terrorists, unless I'm missing something.
This leads to the second question: why don't NATO declare them terrorists and deploy its forces in Crimea as an anti-terrorist action? Because of Putin's denials that would not be an action against Russia but an action against terrorists. That would put Putin in a very awkward position: in order to prevent NATO from engaging these forces he would have to aknowledge that they are a part of Russian Army. The reversal of Russia's statement would in turn reduce his standing in the region, which is the main point of his multiple salvos at the neighbors I believe.