Water-closet n. : A privy; especially, a privy furnished with a contrivance for introducing a stream of water to cleanse it.

Example:
I want to add another water closet, back to back with an existing one. Will the existing 3" drain/vent work?

Taxonomy
toilet, flush, bowl, valve, tank, flush toilet, design, siphon, flush valve, flushometer, cistern, toilet bowl, waste, mechanism, flapper, pressure, valve system, the user.

Defination from the web:
... American plumbing codes still refer to a toilet as a "Water Closet" or a "WC" ...