Also in real estate agents' jargon, at least in the US, half bath means a bathroom which has only a toilet and sink, but no bathtub or shower stall.Guzh is partly right.
People selling houses often speak of a "bath" to mean a bathroom.
I'm not sure of anyone else uses this word in this way.
If I ask my wife where the XYZ is, she will say "Have you looked in the bathroom?", not "Have you looked in the bath?"
It's estate agents' shorthand.