What is an underwater volcano with a flat top referred to as?

An underwater volcano with a flat top is referred to as a guyot. This term specifically describes a geological formation that occurs when a volcanic island is eroded over time by wave action, leading to its summit being flattened. As the surface of the ocean rises or as tectonic plates shift, these submerged flat-topped structures remain at considerable depths in the ocean. Guyots are typically found in deeper ocean basins and are significant in studying the geological history of the ocean floor.

In contrast, an atoll is a ring-shaped coral reef that encircles a lagoon and is usually formed around a submerged volcanic island. A seamount is an underwater mountain but does not specifically denote a flat-topped formation, as it can come in various shapes. A hydrothermal vent is a different feature entirely, characterized by the release of geothermally heated water from the ocean floor, often associated with tectonic activity but not a volcanic structure with a flat top.