Transmitting heat or thermal radiation easily; allowing heat to pass through; diathermal or heat-permeable.
From Greek 'dia-' (through) + 'thermē' (heat) + '-ous' (adjective suffix), a 19th-century technical term in physics for thermal properties.
The opposite of diathermous would be athermanous (heat-blocking), and interestingly, some modern materials are selectively diathermous—they transmit certain heat wavelengths while blocking others, which is how thermal clothing can keep you warm while still feeling breathable.
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