A half bath, in real estate lingo, is just a toilet. A bit ironic in that a half bath really has no bath at all. Maybe a sink, but no shower or bathtub.
I've seen some houses with an inexplicable toilet in odd places, just so the house can be considered to have one-and-a-half baths. Like a toilet in the basement with no walls around it.
See also full bath and three-quarters bath.