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.