Wiktionary
n. (context architecture English) The floor of a building where the principle bedrooms and main reception rooms are found. Very often on the first or second floor in older buildings.
Wikipedia
The piano nobile ( Italian, "noble floor" or "noble level", also sometimes referred to by the corresponding French term, bel étage) is the principal floor of a large house, usually built in one of the styles of Classical Renaissance architecture. This floor contains the principal reception and bedrooms of the house.
The piano nobile is often the first (European terminology, 2nd floor in US terms) or sometimes the second storey, located above a ground floor (often rusticated) containing minor rooms and service rooms. The reasons for this were so the rooms would have finer views, and more practically to avoid the dampness and odors of the street level. This is especially true in Venice where the piano nobile of the many palazzi is especially obvious from the exterior by virtue of its larger windows and balconies and open loggias. Examples of this are Ca' Foscari, Ca' d'Oro, Ca' Vendramin Calergi, and Palazzo Barbarigo.
Larger windows than those on other floors are usually the most obvious feature of the piano nobile. Often in England and Italy the piano nobile is reached by an ornate outer staircase, which negated the need for the inhabitants of this floor to enter the house by the servant's floor below. Kedleston Hall is an example of this in England, as is Villa Capra in Italy.
Most houses contained a secondary floor above the piano nobile which contained more intimate withdrawing and bedrooms for private use by the family of the house when no honoured guests were present. Above this floor would often be an attic floor containing staff bedrooms.
Usage examples of "piano nobile".
The staircase led up to the piano nobile which seemed suspended between sky and earth by rows of slender Byzantine pillars.
Even though he lived in it only a few months of the year, just after the war Bulovius had taken over the piano nobile of a sizable palazzo a stone's throw from the Piazza Navona.