General terms:
* Seamount: A general term for any underwater mountain rising at least 1000 meters from the seafloor.
* Submarine volcano: This term emphasizes the volcanic origin.
* Underwater mountain: A simple and straightforward description.
More specific terms:
* Guyot: A flat-topped seamount, often formed by erosion.
* Volcanic island: If the underwater mountain reaches the surface of the water.
* Hydrothermal vent: If the volcanic activity is specifically associated with hot springs.
The best term depends on the specific characteristics of the underwater volcano you are referring to and the context of your writing.