A soldier is one who fights as part of an organised, land-based armed force. A soldier can be an enlisted person, a non-commissioned officer, or an officer.
The word soldier derives from the Middle English word soudeour, from Old French soudeer or soudeour, meaning mercenary, from soudee, meaning shilling's worth or wage, from sou or soud, shilling. The word is also related to the Medieval Latin soldarius, meaning soldier (literally, "one having pay"). These words ultimately derive from the Late Latin word solidus, referring to an Ancient Roman coin used in the Byzantine Empire.
In most armed forces use of the word 'soldier' has taken on a more general meaning due to the increasing specialization of military occupations that require different areas of knowledge and skill-sets. As a result, 'soldiers' are referred to by names or ranks which reflect an individual's military occupation specialty arm, service, or branch of military employment, their type of unit, or operational employment or technical use such as: trooper, tanker (a member of tank crew), commando, dragoon, infantryman, artilleryman, marine, paratrooper, grenadier, ranger, sniper, engineer, sapper, craftsman, signaller, medic, or a gunner.
In many countries soldiers serving in specific occupations are referred to by terms other than their occupational name. For example, military police personnel in the British Army are known as "red caps" because of the colour of their caps (and berets).
Infantry are sometimes called "grunts" (in the United States Army or United States Marine Corps) or "squaddies" (in the British Army), while US Army artillery crews, or "gunners," are sometimes referred to as "redlegs", from the service branch color for artillery. U.S. soldiers are often called "G.I.s" (short for the term "General Issue"). Members of the Marine Corps are typically referred to as "marines" rather than "soldiers".
In the United States, the term warfighter is often used to refer collectively to all whose job it is to do the actual fighting, although in 2011 the U.S. Army officially started calling its combat personnel soldiers instead of warfighters, in part to avoid confusion among "warfighters" assigned to peace-keeping or other types of duties beyond combat activities. The army has not completely phased out this terminology and still uses "warfighter" in various contexts such as the Project Manager Warfighter Information Network-Tactical.
French Marine Infantry are called marsouins (French: porpoises) because of their amphibious role. Military units in most armies have nicknames of this type, arising either from items of distinctive uniform, some historical connotation or rivalry between branches or regiments.