Well the term “Partisans” is usually 9 times out of 10 used in the context of structured, well organized, Communist organizations fighting behind the lines. The key term there is “Communist”, and of course “well organized”. So if you talk about Partisans in World War II, most World War II Historians and History buffs will actually think you are strictly talking about Tito’s Yugoslav Communist Partisans, Or thinking of the various Soviet Partisan units operating behind the lines of German advance, specifically in Belorussia. Both organizations may not have started as a well –armed, and well organized outfits, but as the war progressed they became highly organized, well trained, well-armed, and well supplied. To a lesser extent, other Communist Resistance movements were also called “Partisans”, simply because they were Communist, not necessarily how large or well organized they were. The best example of this was the Italian Partisans in Northern Italy at the end of World War II, who captured and killed Benito Mussolini and his mistress trying to escape to Austria. Historians refer to resistance, guerilla movements as “Partisan” when they are being politically Partisan (in all of the aforementioned cases, these resistance movements were all Communist).
Historians rarely refer to non-Communist Resistance movements as “Partisans”. For example, The Resistance movements in France were referred to as the aforementioned “Resistance”, but also went by the moniker of Maquis. The Maquis was a loosely organized, amalgamation of various Resistance movements within France, including Communists, but also Democratic, Socialists etc… So in the sense of the French Resistance, Resistance works better than the term “Partisan”, because the French Resistance was not “Partisan”, they were not politically united, only national so – and even then oftentimes betrayed other Resistance movements to the local Vichy Police or the Nazi Gestapo.

Another example of this divergence is in the Yugoslavian partisan war. Yugoslavia, much like how things dissolved in the 1990s, was reminiscent of what happened in World War II. The Yugoslav Partisans under Josip Braz Tito were Communist, and tried uniting the Yugoslavs people (Serbs, Croats, Bosniaks etc…), but predominantly attracted Serbian Communists, and Bosniak Communists. There were other highly effective resistance movements in the shattered Yugoslavia, such as the Chetniks. The Chetniks were formed by Serbian Monarchist and Serbian Nationalists under General Draža Mihailović. The Chetniks waged war against the Nazis, Italians, and the Partisans… This would become a major problem for the various world leaders. The Chetniks were technically the defacto continuation of the Serbian Army, and Government (being directed by the Serbian Monarchy that had fled to England). However, the Chetniks waged open war against the Partisans – because they hated Communists. So in a sense the Yugoslav Guerilla War during World War II was very complex and multifaceted. Both the Chetniks and the Partisans fought against eachother, but also fought against the Germans, Italians and locally raised Fascist Militia units. However, because the British Special Operations Executive (SOE) was highly infiltrated with ardent Communists, the SOE almost exclusively sent weapons, arms, munitions, and supplies to the Partisans. The British refused to acknowledge Chetnik claims to Serbia, and delegitimized the Chetniks as a formal resistance movement. Of course as the war stretched on the Chetniks did some double dipping and some were supplied arms, or given free pass by the Germans and Italians (who in turn loved the fact that the thorn in their side [Yugoslavia], was fighting eachother). But to the Chetniks’ defense, they only double dipped when necessary, and after being delegitimized by the west, in favor of Tito’s Partisan Army. Even after World War II the Chetniks fought on, until eventually being forced to capitulate and surrender to Tito. Their leadership was given trial and executed.

The Polish Underground, or more correctly termed Armia Krajowa (Home Army) was probably the most effective, and largest non-Communist resistance movement. When westerners envision resistance to Nazi occupation and rule they imagine some swarthy and sultry French Woman or Man, with a beret and leather jacket carrying a Sten Mk III sub Machinegun. In reality, they should be envisioning similar, but Polish, using captured German weaponry. The Home Army, or more commonly referred to as the Polish Underground, dwarfed the French resistance several fold. The Home Army was also very well organized, and tried uniting the various Polish resistance factions (including Communist and the various Jewish resistance groups). They were given support by the British, and were also acknowledge by the British. Their Government was exiled in England, but still directed raids, assassinations, attacks etc… However, as the war progressed, and Poland was to fall under the Soviet’s sphere of influence, the British and the West started to slowly distant themselves from the Home Army, and the Polish Exiled Government. The Polish Underground revolted in Warsaw in 1944, as the Red Army was fast approaching. However, Stalin, who had always hated the Polish (and specifically wanted Poland to be Communist) decided to slow advances towards Warsaw, and redirect his forces, thus leaving the Warsaw Uprising problem for the Germans to put down. Hey why not, kill two birds with one stone? Get your enemy to do your dirty work. The Soviet Union may have had the most casualties during World War II, but Poland certainly lost the most. The war was started over Nazi Germany and the USSR violating their sovereignty, and yet ended with a puppet Communist regime put in place, and the nation falling behind an Iron Curtin.

Anyway, so to answer your question directly: Historians only refer to politically solidified (and these were almost entirely Communist) resistance movements as “Partisans”. Otherwise, resistance movements that were democratic, amalgamated, disjointed, or were otherwise mostly non-Communist, are called various other things, such as the Underground, or Resistance etc…