A soldier is a person who has enlisted with (or was drafted into) the armed forces of a sovereign country and has undergone training and received equipment to defend that country or its "interests". Soldiers usually act in strictly hierarchical societies called armies. Infantry are soldiers who specialize in ground combat. Not all soldiers are infantry, but all infantry are soldiers.
A pre-modern soldier is also called a warrior, often with the additional qualities of valour and honour, proclaimed by himself or by his peer.