An army, ground force or land force is an armed 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 country. It may also include aviation assets by possessing an army aviation component. Within a national military force, the word army may also mean a field army.

In some countries, such as France and China, the term "army", especially in its plural form "armies", has the broader meaning of armed forces as a whole, while retaining the colloquial sense of land forces. To differentiate the colloquial army from the formal concept of military force, the term is qualified, for example in France the land force is called Armée de terre, meaning Land Army, and the air and space force is called Armée de l'Air et de l’Espace, meaning Air and Space Army. The naval force, although not using the term "army", is also included in the broad sense of the term "armies" — thus the French Navy is an integral component of the collective French Armies (French Armed Forces) under the Ministry of the Armies. A similar pattern is seen in China, with the People's Liberation Army (PLA) being the overall military, the land force being the PLA Ground Force, and so forth for the PLA Air Force, the PLA Navy, and other branches. 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).