An onsen (温泉?) 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 (露天風呂 or 野天風呂, roten-buro or noten-buro?) and indoor baths. Baths may be either public run by a municipality or private (内湯, uchiyu?) often run as part of a hotel, ryokan or bed and breakfast (民宿, minshuku?).

Onsen are a central feature of Japanese tourism often found out in the countryside but there are a number of popular establishments still found within major cities. They are a major tourist attraction drawing Japanese couples, families or company groups who want to get away from the hectic life of the city to relax. Japanese often talk of the virtues of "naked communion" (裸の付き合い, hadaka no tsukiai?) for breaking down barriers and getting to know people in the relaxed homey atmosphere of a ryokan with an attached onsen. Japanese television channels often feature special programs about local onsens.

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 sentō, 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 (打たせ湯?).

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 sentō but single-sex bathing has become legalized as the norm since the opening of Japan to the West during the Meiji period. Mixed bathing (混浴, kon'yoku?) persists at some special onsen 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.

Read more about Onsen:  Therapy, Risks, Incidents, Selected Onsen

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