Cistern

WC Cistern

The modern toilet utilises a cistern to reserve and hold the correct amount of clean water required to flush the toilet bowl where the cistern may be mounted directly on the toilet bowl with no intermediate pipe. In this arrangement, the flush mechanism (lever or push button) is usually mounted on the cistern. Concealed cistern toilets, where the cistern is built into the wall behind the toilet, are also available.

WIKIPEDIA ENTRY FOR CISTERN.

Toilet Cistern Picture