Knoll (oceanography)
In marine geology, a knoll is defined as a rounded underwater hill, not exceeding 1000 meters in height. Any rounded underwater features exceeding that height are referred to as seamounts. They are believed to cover around 16.3% of the world's seafloor. Examples 
 Orphan Knoll
 Graveyard Seamounts

References 
Oceanographical terminology
Seamounts