The measure of how effectively a surface emits thermal radiation, expressed as a ratio compared to a perfect black body.
From emissive (able to emit) plus the suffix -ity, which creates abstract nouns expressing degree or quality, similar to conductivity or reactivity.
Emissivity is why infrared cameras can 'see' heat in different ways depending on surface material—a reflective aluminum foil might have emissivity around 0.05, while human skin has about 0.98, making us glow in thermal imaging.
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