The measure of how easily a substance diffuses through another medium; the coefficient representing the rate of diffusion.
From diffusive + -ity (suffix forming nouns of state or condition). Diffusivity is a technical term that became standard in 19th-century physics and chemistry.
Thermal diffusivity explains why aluminum heats up faster than wood—it's not just about conduction, it's about how fast heat energy spreads through the material's structure at the molecular level.
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