Zashiki (literally, sitting mat) is the place where (in the medieval times) the floor was covered with a mat or carpet so as to, literally, sit down.
In the case of today's conventional home, the Zashiki is the finest Japanese-style room. It is bright with sunshine and well-ventilated, and it may have a tokonoma (a Japanese-style alcove). The room is used to conduct rituals on ceremonial occasions for the coming of age, marriage, funeral and ancestral worship, and annual ceremonies. In addition, it often functions as a reception room and a guestroom. See an article on the spread of Shoin-zukuri style (a style of Japanese domestic architecture that includes a tokonoma).
A Zashiki is a large Japanese-style room in a restaurant, hotels or the like, which functions as a banquet room.
A Zashiki is a task for geisha (more precisely, geigi or geiko) to attend banquets. The task includes entertaining customers with music and dance and pouring sake for them. A common expression in which the term "zashiki" is used in this sense is "(Geisha) get called in for o-zashiki" (in Japanese, the prefix 'o' is added to a word such as "zashiki" to be polite).
The ozashiki songs belong to zokkyoku (popular melody) sung to samisen accompaniment, including nagauta (long epic songs), hauta (short love songs), kouta (ballads) and kamigata uta (songs from the Kyoto-Osaka area). They allegedly originated from dance music performed within a kabuki play, Kiyomoto (kiyomotobushi; a school of joruri recitation in which the voices are pitched slightly higher than usual) and the like, and gained popularity from the Bunka-Bunsei period (1803-1830). The ozashiki songs were sung by geisha (more precisely, geigi or geiko) and yujo (prostitutes) at ozashiki, becoming popular among the common people.
As for the origin of kayokyoku (Japanese popular songs, distinguished from J-pop), a leading theory advocated by Otojiro KAWAKAMI is that it originated from enzetsu-ka (literally, speech songs, political satire taking the form of songs) which developed into enka. When record companies were founded one after another in the early Showa period, there emerged many singers who were on each record label, such as Tamaki TOKUYAMA, Chiyako SATO, Ichiro FUJIYAMA and noriko AWAYA. Among these singers who had studied singing at school, there were also singers who were originally geisha, such as Katsutaro KOUTA, Ichimaru, Koume AKASAKA and Michiyakko. There were many popular songs which followed in the wake of ozashiki songs.
Particularly, around 1950, fancy Japanese-style restaurants started prosper all over the country in a Korean War-related economic boom. In the popular song world, many song records under the category of ozashiki songs were released to catch onto the popularity of those restaurants. Singers to sing the ozashiki songs made their debut, including Hanko KAGURAZAKA and Yukie KUBO. Songs, such as "Geisha Waltz" and "Tonko Bushi" dominated the popular song world in Japan.