An onkian was a temperature unit used within the Romulan Empire.

Twelve onkians were equivalent to a temperature on the cold side of the Celsius metric system. Admiral Alidar Jarok, after requesting political asylum on the USS Enterprise-D, ordered water with a temperature of twelve onkians from the replicator in his guest quarters; the units were not recognized by the computer. (TNG: "The Defector")

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