Strictly speaking North buffalo are bison But what do we call European bison?

Strictly speaking, North American bison are not classified as buffalo. The term "buffalo" typically refers to species of the large wild bovines in the genus Bubalus found in Africa and Asia. North American bison belong to the genus Bison and are more closely related to European bison (Bison bonasus) than to water buffalo or African buffalo. Therefore, European bison are also not truly buffalo and should be referred to as either European bison or wisent.