The Collaborative International Dictionary of English v.0.48:
Tank ship \Tank ship\, Tank vessel \Tank vessel\ . (Naut.) A vessel fitted with tanks for the carrying of oil or other liquid in bulk; -- called also tanker. A tank ship of very large capacity is called a supertanker. [Webster 1913 Suppl. +PJC]WordNet (r) 3.0 (2006):
supertanker n 1: the largest class of oil tankers