Partisans are members of an irregular military force, formed to oppose control of an area by a foreign power or an army of occupation, by some kind of insurgent activity.
Partisans may be unlawful combatants, if they engage in armed conflict in violation of the internationally agreed on laws of war, which means that they are not protected by such laws and rights. The occupier may consider partisans to be "terrorists", the partisans may consider themselves to be "freedom fighters".
The term has in particular been applied to insurgents during WWII. See National Socialist Germany and partisans/resistance movements.