diffusivity

This term is a specialized technical noun used primarily in physics, chemistry, and materials science. It describes a specific physical property of a medium rather than the act of diffusion itself. While diffusion refers to the process of spreading, diffusivity is the quantitative coefficient that measures the efficiency or speed of that process.

Because it represents a physical constant or a measurable property of a substance, it is treated as an uncountable noun in scientific discourse. It is almost never used in plural form, as it refers to a characteristic value rather than a countable object.