Relating to or involving the process of diffusion; caused by or characteristic of diffusion.
From diffusion + -al (Latin suffix meaning 'relating to'). The -al suffix is one of the most productive in English for creating adjectives from nouns, especially in scientific terminology.
When scientists study 'diffusional flow' in the ocean or atmosphere, they're describing the random molecular wandering that, averaged across billions of particles, creates predictable currents.
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