Volcanic disasters take diverse forms and volcanic eruptions are difficult to predict. In order to utilize information to prevent volcanic disasters it is undoubtedly important for inhabitants, local authorities and disaster professionals to have a good understanding of each other. To advance this mutual understanding, volcanic phenomena should be described in a unified manner. The terms “volcanic mudflow”,“mudflow” and “debris flow ” all refer to phenomena in which a flow with densely mixed pyroclastic materials and water runs down on a mountainside. However, these three terms are still controversial in their meanings. In this paper the author abstracts descriptions on “volcanic mudflow”,“mudflow” and “debris flow” from historical literature of volcanology, Sabo engineering, and related fields in order to clarify the history of usage of these terms. Their appropriate usages are discussed from the viewpoint of social demands for volcanic disaster prevention while considering the present efforts actually being made in Japan. As a result, it is proposed that the phenomenon directly caused by volcanic eruptions should be called “volcanic mudflow” and that the phenomenon caused by rainfall after eruptions should be called “debris flow” or “mudflow”, when these terms are used in the field of volcanic disaster prevention. However, the way of defining volcanic eruptions or volcanic activities with tangible expressions still needs further discussion. The phenomena triggered by the direct extrusion of muddy materials from a crater should also be discussed so that they can be categorized depending on the type of volcanic mudflow involved.