therm
noun
any of several units of heat, as one equivalent to 1000 large calories or 100,000 British thermal units.
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Such zones or divisions of the land, ocean, or atmosphere, which have an equal degree of mean annual warmth, are said to be isothermal, from ισος, isos, equal, and θερμη, therme, heat.
From Principles of Geology or, The Modern Changes of the Earth and its Inhabitants Considered as Illustrative of Geology by Sir Charles Lyell