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):

tank ship
    n 1: a cargo ship designed to carry crude oil in bulk [syn: oil
         tanker, oiler, tanker, tank ship]