thumb|Outdoor onsen at Nakanoshima in [[Nachikatsuura, Wakayama]]
thumb|thumbtime=9|A video showing the stool and shower used for cleaning off, an inside pool and an outside pool
In Japan, are hot springs and the bathing facilities and traditional inns around them. There are approximately 25,000 hot spring sources throughout Japan, and approximately 3,000 establishments use naturally hot water from these geothermally heated springs.
' may be either or . Traditionally, were located outdoors, although many inns have now built indoor bathing facilities as well. Nowadays, as most households have their own baths, the number of traditional public baths has decreased, but the number and popularity of have increased since the end of Second World War. Baths may be either publicly run by a municipality or privately, often connecting to a lodging establishment such as a hotel, ryokan, or minshuku. thumb|Indoor at [[Asamushi Onsen]]
The presence of an is often indicated on signs and maps by the symbol ♨, the kanji (yu, meaning "hot water"), or the simpler phonetic hiragana character ゆ (yu).
Definition
According to the Japanese , is defined as "hot water, mineral water, and water vapor or other gas (excluding natural gas of which the principal component is hydrocarbon) gushing from underground". The law states that mineralized hot spring water that feeds an must be at least originating at a depth of at least , and contain specified amounts of minerals such as sulphur, sodium, iron, or magnesium. in part because the specific minerals found in the water have been thought to provide health benefits. Types include , , , and .
History
Exactly when humans first began bathing in in Japan is unknown, but historical records show it has a history of at least about 1,300 years. Many of the earliest records of bathing document Japanese Emperors or members of the imperial family staying at for long periods of time. At the time, were thought to be sacred and to have healing properties. It is now believed those who traveled to in these early records were seeking cures to disease or other ailments by bathing in the water. Some of the oldest in Japan are considered to be Dogo Onsen in Ehime Prefecture, Shirahama Onsen in Wakayama Prefecture, and Arima Onsen in Hyogo Prefecture.
In modern times, traveling to is a popular form of domestic tourism in Japan. Depending on the prefecture and local ordinances, children seven years old and younger may be exempt from this ban.
In the Japanese language, means Woman and means Man. The characters and (pronounced yu) stands for Hot water, but also for a Bathing or . Generally, there is a separation called curtains.
Etiquette
thumb|Baskets
Onsen are a location where considerable offense can be caused if the proper social etiquette is not observed. The general procedure and expected etiquette closely resemble those used at a sentō but with some minor adjustments. Onsen are typically, but not always, found within ryokan or hotels and only available to guests, though they may offer certain hours where outsiders may purchase day passes to enter. Standalone onsen open to the public also exist. Onsen are considered more of a special experience than the sento, which is used for common bathing. Due to their integrated location within a hotel, an entry attendant is rare and bathers will usually wear the hotel-provided yukata and indoor slippers to walk from their rooms to the onsen. Towels are provided to guests at the entry, and shower stations and vanities are fully stocked with toiletries and conveniences for bather's use, instead of guests being required to bring their own or purchase such items separately. Guests should not take wet towels or toiletries back with them or hoard the complimentary items. While it is common after a bath in the sento to quickly re-shower before leaving, many onsen bathers skip this second shower to keep the minerals from the hot spring on their skin. Onsen within ryokan or hotels are generally kept open 24 hours a day, or are only closed for a brief daily cleaning period, to allow guests to freely bathe in the middle of the night if the whim takes them. Popular, private, or specialized baths may require reservations or additional fees.
Ensuring cleanliness
As at a sentō, at an , all guests are expected to wash and rinse themselves thoroughly before entering the hot water. Bathing stations are equipped with stools, faucets, rinsing basins, and complimentary toiletries such as soap and shampoo; nearly all onsen also provide removable shower heads for rinsing convenience. Entering the while still dirty or with traces of soap on the body is socially unacceptable.
One's hair should never touch the bath water and should be worn up.
Even though onsen can be a luxury experience and environment, guests are still expected to clean up after themselves by staying tidy and rinsing off their shower area for the next guest.
Swimsuits
Guests are not allowed to wear swimsuits in the baths or showers and must be completely nude.
The only time swimsuits are allowed and required are in specific modern pools which allow both men and women to use them together, but many of these mixed resemble more of a pool or waterpark than the traditional idea of an .
Towel
guests generally have a small towel with them to use as a drying cloth, since re-entering the changing area while dripping wet is a faux-pas. The towel can also provide a modicum of modesty when walking between the washing area and the baths by holding it loosely in front of your genitals. Though many TV shows depict characters wearing a towel in the bath, this is a television invention used for actor modesty and wearing towels into the bath is forbidden. It is against the rules to immerse or dip towels in the bath water, since this is considered unclean. 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 to keep them out of the water.
Tattoos
By 2015, around half (56%) of operators had banned bathers with tattoos from using their facilities. The original reason for the tattoo ban was to keep out yakuza and members of other crime gangs who traditionally have elaborate full-body decoration. In 2016, the Japan Tourism Agency urged operators to allow foreign customers with tattoos to use the on a conditional basis in order to accommodate the increasing number of foreign tourists. On the other hand, operators wanted foreign customers to enjoy the , but were concerned that allowing foreign customers with tattoos to use the would prevent them from keeping yakuza out of the .
However, tattoo-friendly do exist. A 2015 study by the Japan National Tourism Organisation found that more than 30% of operators at hotels and inns across the country will not turn someone with a tattoo away; another 13% said they would grant access to a tattooed guest under certain conditions, such as having the tattoo covered up.
With the increase in foreign customers due to growing tourism, some that previously banned tattoos are loosening their rules to allow guests with small tattoos to enter, provided they cover their tattoos with a patch or sticking plaster.
Benefits
There are various health benefits attributed to bathing in . are particularly renowned for their relaxing and therapeutic effects.
A 2014 study conducted in Beppu, a city famous for its , found that regular bathing in has various health benefits such as lowering blood pressure, improving circulation and cardiovascular health, reducing chronic pain and fatigue, and lowering the rate of depression.
Water at some facilities is drinkable. water should only be consumed if it is confirmed to be safe to drink by the prefecture. Although millions of Japanese bathe in every year with few noticeable side effects, there are still potential side effects to usage, such as aggravating high blood pressure or heart disease.
Legionella bacteria have been found in some with poor sanitation. For example, 295 people were infected with Legionella and seven died at an in Miyazaki Prefecture in 2002.