A powder room is a small bathroom, usually under 50 square feet, designed for general use by both residents of the house and guests. It’s usually located on the ground level, near common areas.
At its most basic, a powder room consists of a toilet and a sink (after all, you don’texpect your guests to take a shower or bath). A powder room allows guests to relieve themselves and wash their hands.
Contrast this with a guest bathroom–for overnight guests–which may be adjoining a guest bedroom, consisting of a toilet, sink, and a bathtub or shower.