公国，是一种君主制国家，指封建制下公爵或女公爵的封国。欧洲封建制中的公国拉丁文称为Ducatus。中国封建制中也有公国，是中国公爵的封地。

公国是由公爵获皇帝、国王等的赐封的领地；被国王赐封的领地给公爵管辖的就称为公国。

The stem duchies (German:Stammesherzogtümer, from stamm, meaning "tribe", and in reference to the Germanic tribes of the Franks, Saxons, Bavarians and Swabians) were the constituent duchies of the kingdom of Germany at the time of the extinction of the Carolingian dynasty (the death of Louis the Child in 911) and the transitional period leading to the formation of the Holy Roman Empire later in the 10th century. The Carolingians had dissolved the original tribal duchies of the Frankish Empire in the 8th century. As the Carolingian Empire declined in the late 9th century, the old tribal areas assumed new identities as the subdivisions of the realm. These are the five stem duchies (sometimes also called "younger stem duchies" in reference to the pre-Carolingian tribal duchies): Bavaria, Franconia, Lotharingia, Saxony and Swabia (Alemannia). The stem duchies were retained as the major divisions of Germany under the Salian dynasty, but they became increasingly obsolete during the early high medieval period under the Hohenstaufen, and they were finally abolished in 1180 by Frederick Barbarossa in favour of more numerous territorial duchies.