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.