What floor is the first floor? The answer depends on what country you are in.
If you are in the US, the first floor is usually the same as that in Japan, namely, the floor level to the ground. In Europe however, the First Floor is sometimes equivalent to what we Americans and Japanese would call the second floor. There, the first floor is called the “ground floor”.
According to Wikipedia:
This convention can be traced back to Medieval European usage. In countries that use this system, the floor at ground level is usuallyreferred to by a special name, usually translating as "ground floor" or equivalent. For example, Erdgeschoss ("ground floor") in Germany, piano terra (lit. "ground floor") in Italy, begane grond (lit. "troddenground") in the Netherlands, planta baja or planta baixa ("bottom floor") in Spain, beheko solairua in Basque, andar térreo ("ground floor") in Brazil, rés-do-chão ("adjacent to the ground") in Portugal, földszint ("ground level") in Hungary (although in Budapest the "félemelet" or "half floor" is an extra level between the ground and first floors, apparently the result of a tax evasion trick in the 19th century), rez-de-chaussée (from French street level, where "rez" is the old French of "ras", meaning "scraped".) in France, parter in Poland and Romania, prízemie ("by the ground") in Slovakia, and pritličje ("close to the ground") in Slovenia. In some countries that use this scheme, the higher floors may be explicitly qualified as being above the ground level, such as in Slovenian "prvo nadstropje" (literally "first floor above ceiling (of the ground storey)").
In Spain, the level above ground level (the mezzanine) is sometimes called "entresuelo" (entresòl in Catalan, etc., which literally means "interfloor"), and elevators may skip it. The next level is sometimes called "principal". The "first floor" can therefore be three levels above ground level. In Italy, in the ancient palaces the first floor is called piano nobile ("noble floor"), since the noble owners of the palace lived there.
In France, there are two distinct names for storeys in buildings which have two "ground floors" at different levels (on two opposite faces, usually). The lower one is called rez-de-chaussée (lit. "adjacent to the road"), the upper one is rez-de-jardin (lit. "adjacent to the garden"). The same differentiation is used as well in some buildings in Croatia. The lower level is called razizemlje (abbr. RA), and the upper prizemlje (PR). If there's only one ground floor, it's called prizemlje. The latter usage is standard for smaller buildings, such as single-family homes.