An onsen is a term for hot springs in the Japanese language, though the term is often used to describe the bathing facilities and inns around the hot springs. As a volcanically active country, Japan has thousands of onsen scattered along its length and breadth. Onsen were traditionally used as public bathing places and today play a central role in directing Japanese domestic tourism.
Onsen come in many types and shapes, including outdoor (rotenburo or notenburo) and indoor baths. Baths may be either public run by a municipality or private (uchiyu) often run as part of a hotel, ryokan or bed and breakfast (minshuku).
The presence of an onsen is often indicated on signs and maps by the symbol ♨ or the kanji, 湯 (yu, meaning "hot water"). Sometimes the simpler hiragana character ゆ (yu) is used, to be understandable to younger children.
Traditionally, onsen were located outdoors, although a large number of inns have now built indoor bathing facilities as well. Onsen by definition use naturally hot water from geothermally heated springs. Onsen should be differentiated from sento, indoor public bath houses where the baths are filled with heated tap water. The legal definition of an onsen includes that its water must contain at least one of 19 designated chemical elements, including radon and metabolic acid and be 25 °C or warmer before being reheated. Stratifications exist for waters of different temperatures. Major onsen resort hotels often feature a wide variety of themed spa baths and artificial waterfalls in the bathing area utaseyu.
Onsen water is believed to have healing powers derived from its mineral content. A particular onsen may feature several different baths, each with water with a different mineral composition. The outdoor bath tubs are most often made from Japanese cypress, marble or granite, while indoor tubs may be made with tile, acrylic glass or stainless steel. Different onsen also boast about their different waters or mineral compositions, plus what healing properties these may contain. Other services like massages may be offered.
Traditionally, men and women bathed together at the onsen and sento but single-sex bathing has become legalized as the norm since the opening of Japan to the West during the Meiji-era. Mixed-sex bathing persists at some special onsen (konyoku) in the rural areas of Japan, which usually also provide the option of separate "women-only" baths or different hours for the two sexes. Children of either sex may be seen in both the men's and the women's baths.
People often travel to onsen with work colleagues, friends, couples or their families.
Ensuring cleanliness: At an onsen, as at a sento, all guests are expected to wash their bodies and rinse themselves thoroughly before entering the hot water. Bathing stations are equipped with stools, faucets, wooden buckets, and toiletries such as soap and shampoo; nearly all onsen also provide removable shower heads for bathing convenience. Entering the onsen while still dirty or with traces of soap on the body is socially unacceptable.
Swimsuits: Bathers are not normally allowed to wear swimsuits in the baths. However, some modern onsen with more of a waterpark atmosphere require their guests to wear a swimming suit in their mixed baths.
Towels: Onsen guests generally bring a small towel with them to use as a wash cloth. The towel can also provide a modicum of modesty when walking between the washing area and the baths. Some onsen allow one to wear the towel into the baths, while others have posted signs prohibiting this, saying that it makes it harder to clean the bath. It is sometimes against the rules to immerse or dip towels in the onsen bath water, since this can be considered unclean. In this latter case, people normally set their towels off to the side of the water when enjoying the baths, or place their folded towels on top of their heads.
Noise: Onsen vary from quiet to noisy, some play piped music and often feature gushing fountains. Bathers will engage in conversation in this relaxed situation. There are usually prohibitions against rowdiness in the washing and bathing areas. A small amount of excess energy and splashing around is usually tolerated from children, however.
Tattoos: Many onsen ban bathers with tattoos, which in Japan are traditionally perceived as a badge of criminality—Yakuza traditionally have elaborate tattoos. Despite this background reason, the rule is often enforced strictly against all, including foreigners, women, and even when tattoos are small and "peaceful".