The term "Roman soldier" is synonymous with a legionnaire, a soldier of the Roman Empire's ancient armies. There are many other terms that have been used over time to refer to these soldiers, including centurions, auxilia, and legionaries. The centurions were the officers who commanded a legion or century of troops, while the auxilia were non-citizen soldiers who served in support of the legions. Legionaries were the backbone of the Roman army, and made up the majority of the fighting force. These various synonyms for Roman soldiers highlight the different types of soldiers that made up the Roman army, as well as the complex organization and hierarchy of the Roman military.