Marine

A branch of a country's navy which fights on land. Often designated a corps, not a service. Marines are naval infantry, specialising in fighting at sea on ships, forward deployment, naval security and amphibious assault.

Originated from the fact that it was an act of war for a country to send its army into another country but not to send its navy.

Note that some countries do not differentiate branches of their armed forces, as Israeli Defense Force, Canadian Armed Forces.