What is Army?

Legal Definition
An army (from Latin arma "arms, weapons" via Old French armée, "armed" (feminine)) or ground force is a fighting force that fights primarily on land. In the broadest sense, it is the land-based military branch, service branch or armed service of a nation or state. It may also include other branches of the military such as the air force via means of aviation corps. Within a national military force, the word army may also mean a field army. They differ from army reserves who are activated only during such times as war or natural disasters.

In several countries, the army is officially called the Land Army to differentiate it from an air force called the Air Army, notably France. In such countries, the word "army" on its own retains its connotation of a land force in common usage. The current largest army in the world, by number of active troops, is the People's Liberation Army Ground Force of China with 1,600,000 active troops and 510,000 reserve personnel followed by the Indian Army with 1,129,000 active troops and 960,000 reserve personnel.

By convention, irregular military is understood in contrast to regular armies which grew slowly from personal bodyguards or elite militia. Regular in this case refers to standardized doctrines, uniforms, organizations, etc. Regular military can also refer to full-time status (standing army), versus reserve or part-time personnel. Other distinctions may separate statutory forces (established under laws such as the National Defence Act), from de facto "non-statutory" forces such as some guerrilla and revolutionary armies. Armies may also be expeditionary (designed for overseas or international deployment) or fencible (designed for – or restricted to – homeland defence).
-- Wikipedia
Legal Definition
The armed forces of a nation intended for military service on land. "The term 'army' or 'armies' has never been used by congress, so far as I am advised, so as to include the navy or marines, and there is nothing in the act of 1862, or the circumstances which led to its passage, to warrant the conclusion that it was used therein in any other tban its long established and ordinary sense,— the land force, as distinguished from the navy and marines." In re Bailey, 2 Sawy. 205, Fed. Cas. No. 728. But see In re Stewart, 7 Rob. (N. Y.) 636.
-- Black's Law Dictionary