Long ago, the term "hot spring" was coined to describe water that has a high temperature and arises from underground. 　Many believed that it was named by Zhang Heng (AD78-139), who was a politician, literary scholar and natural scientist from China. In Japan, a land of volcanoes, scholars believe that hot springs were known and used by people even before then. 　However, hot spring use was not recorded until the eighth century using the adapted Chinese characters, when books such as Kojiki (The Records of Ancient Matters), Nihonshoki (Chronicles of Japan) and Fudoki (Topography) were compiled.

　A hot spring (or something that was deemed to be a hot spring) was mentioned in the Bungo Province Fudoki (a historical record on Oita Prefecture), in the sections about Hita County, Naoiri County, Oita County and Hayami County. However, the term "Onsen" (hot spring in Japanese) was not used, and instead, it was called "On no Sen" (hot spring), "On Yu" (hot water), "Yu Ga" (hot river), "Yu Sen" (hot spring) and "Yu I" (hot water well) (Okimori, Sato and Yajima, 2008).

　A thousand years later, as Japan transformed into a modern nation, people started to have a different perception of hot springs.　There was a need to introduce an objective definition about hot springs. 　A typical natural science definition would be as follows – “a phenomenon wherein the water that flows out from the underground to the surface that is hotter than the usual groundwater” (Yuhara & Seno, 1969).

　The feel and concept of “warm water” is determined by the degree of the temperature.
The aforementioned temperature of the groundwater is about one to four degrees (°C) higher than the annual mean temperature of the area. The temperature is basically created by the heat from the sun. Therefore, this definition recognizes and asserts that the heat from within the earth, in addition to the heat from the sun, specifically heat related to volcanic activity raises the water temperature.

　This definition is considered rational in terms of natural science. However, the threshold temperature that was determined by this definition differs according to location.　 Therefore, it was difficult to apply this definition in an actual situation. As such, 25°C was then chosen as the threshold temperature to be used across Japan.　 It was selected based on the Japan Pharmaceutical Society Agreement, which was the standard hot spring analysis method before the Pacific War.　 This value was decided based on the annual mean temperature of different regions in Japan. 　However, it was said that the annual mean temperature of Taiwan, which was under Japanese rule at that time, acted as a benchmark. 　When the Hot Spring law was enacted in 1948, the 25°C benchmark was carried over (Hattori, 1959).
　According to the data in the Annual Chronological Scientific Tables published by the National Astronomical Observatory of Japan, the highest annual mean temperature in Japan was in Naha, Okinawa Prefecture, at 22.7°C (mean value of years 1971-2000). 　Therefore, 25°C is considered to be suitable for use across Japan.
　Naturally, this threshold temperature was regulated more strictly in colder areas than in warmer areas. For example, spring water with a temperature of 14°C in an area with an annual mean temperature of 6℃ would not be recognized as a hot spring, even if it was evidently warmed by the heat that was generated from within the earth.

　In order to better address this discrepancy, Fukutomi introduced the idea of tepid hot springs in 1952. This idea classifies hot springs according to their temperature, and the lower limit temperature was proposed to be “the annual mean temperature of the area + 7°C”. 　This "7°C" was set based on the fact that the general geothermal gradient is 3°C/100m, and the assumption that the depth range of the groundwater flowing out to the surface is approximately 100m. Then, that original temperature of 4°C receives an additional 3°C.

Fukutomi’s classification of hot springs:

Springs (cool springs)		< (annual mean temperature of the area + 7°C)
(annual mean temperature of the area + 7°C）		≤ tepid hot springs < 25°C
Hot springs		25°C≤  warm springs  < 40°C
		40°C≤ high-temperature hot springs < boiling point (100°C  at 1 atm)
		Boiling springs: water at the state of boiling .

(Reference)
　At the end of the Edo period, Yoan Udagawa (1798-1846), who was one of the first to introduce modern science to Japan, had divided the types of hot springs into five categories, ranging from high to low temperature hot springs: high-temperature hot springs; hot springs; warm springs; cool springs; and cold springs.

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