A military or military force (n., from Latin militarius, miles "soldier") has seen many different incarnations throughout time. Early armies were most likely men with sharpened sticks and rocks; through time they have included advancements such as men mounting horses, men wielding swords and other metallic weapons, the bow and arrow, siege weapons, the stirrup, to the advance of the musket which form the roots of the armed forces of most nations we know today. In modern times people use motorized vehicles and firearms.

While military can refer to any armed force, it generally refers to a permanent, professional force of soldiers or guerrillas—trained exclusively for the purpose of warfare and should be distinguished from a sanctioned militia or a levy, which are temporary forces— citizen soldiers with less training, who may be "called up" as a reserve force, when a nation mobilizes for total war, or to defend against invasion. The term military is often used to mean an army.

The doctrine that asserts the primacy of a military within a society is called militarism.