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1891, "part of a house arranged to receive the sun's rays," earlier "sundial" (1842), from Latin solarium "sundial," also "a flat housetop," literally "that which is exposed to the sun," from sol "sun" (see sol).
in architecture, any room that is exposed to the sun. While the term may also be applied to the open sunporches or apartments on the roofs of ancient Greek or Roman houses, it is now used especially to designate a room that is enclosed in glass. In such a solarium, three or possibly four walls and sometimes even the ceiling may all be of glass. Often the solarium is a feature of a hospital or sanitarium, a room in which patients may be exposed to sunlight in a controlled environment as part of their therapy.