A military or military force (n., from Latin militarius, miles "soldier") has seen many different incarnations throughout time. Early armies may have been just men with sharpened sticks and rocks; through time they have included advancements such as men mounted on horses, men wielding swords and other metallic weapons, the bow and arrow, siege weapons, 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 vehicles and guns.
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.
Meaning of the Word
As an adjective, "military" is a descriptive property of things related to soldiers and warfare. It also refers to such context dependent terms such as military reserves which may indicate an actual unit deployable on command or the general sense, of a Nation States reserve troops available to or eligible for duty in its armed forces.
In formal British English, "military" as an adjective refers more particularly to matters relating to an army (land forces), as opposed to the naval and air force matters of the other two services.
In American English, "military" as an adjective is more widely used for regulations pertaining to and between military procurement, military transport, military justice, military strength, and military force.