Terminology
The word toire (トイレ?) is an abbreviated form of the English language word "toilet" and is used both for the toilet itself and for the room where it is located. Similarly to the word anime, toire is often mistakenly identified as a French language loan word due to its French-sounding ending.
A common euphemism is otearai (お手洗い?, lit. hand-washing). Strictly speaking, otearai refers to the sink and is actually a loan translation of the word "lavatory". This is similar to the usage in American English of "washroom", which literally means a room where something is washed, and "toilet", which literally refers to the act of self-cleaning. It is also common to see another loan translation, keshōshitsu (化粧室?, lit. powder room), on signs in department stores and supermarkets, as well as accompanying the public toilet pictogram.
The plain word for toilet is benjo (便所?, place of convenience or place of excrement), from the word ben (便?) meaning "convenience" or "excrement", and this word is fairly common. It is often used in elementary schools, public swimming baths, and other such public places, and is not especially impolite, although some may prefer to use a more refined word. In many children's games, a child who is tagged "out" is sent to a special place, such as the middle of a circle, called the "benjo." Japanese has many other words for places reserved for excretory functions, including kawaya (厠) and habakari (憚り), but most are rare or archaic.
The toilet itself—that is, the bowl or in-floor receptacle, the water tank, et cetera—is called benki (便器). The toilet seat is benza (便座). A potty, either for small children or for the elderly or infirm, is called omaru (sometimes written 御虎子).
The Japan Toilet Association celebrates an unofficial Toilet Day on November 10, because in Japan the numbers 11/10 (for the month and the day) can be read as ii-to(ire), which also means "Good Toilet".
Read more about this topic: Toilets In Japan