cistern vs water tank

cistern

noun
  • A reservoir or tank for holding water, especially for catching and holding rainwater for later use. 

  • The vessel surrounding the condenser in a steam engine. 

  • In a flush toilet, the container in which the water used for flushing is held; a toilet tank. 

  • A cisterna. 

water tank

noun
  • A tank (type of large container) used to store water. 

How often have the words cistern and water tank occurred in a corpus of books? (source: Google Ngram Viewer )