Ground floor
Bikermom has info on ground floor, which does not include the fact that while we use "first floor" as a synonym for ground floor, in UK and the continent, the first floor is what we call the second; i.e., the first above the ground floor.
The ground floor does not have to be directly on the ground. Baltimore, where I live, is famous for its row houses, many of which have white marble steps, which every housewife who didn't want to become known as the neighborhood slut faithfully scrubbed every Saturday so they would always be white and gleaming, a custom now practiced mostly by the elderly. The steps are necessary because the ground floor is actually 3 or 4 feet off the street level, to allow small windows in the front and back to provide light in the basement (all such houses have a basement). The basement, in 19th century houses, always contained the kitchen. In most small row houses, the part of the basement not including the laundry tubs, washer, dryer, and furnace has been panelled (knotty pine is the classic treatment), with tile or linoleum floor and drop ceiling to make a family room. The reason for the kitchen in old houses and the family room in later ones being in the basement is the Baltimore climate, which is positively tropical in summer. Those row houses are unbearably hot from mid-June to mid-Sept., so in the days before air conditioning was common, they fixed up the basement since it was much cooler.
