Diffusion
Etymology
From Latin diffusio, "a pouring out, a spreading," from diffundere (dis-, "apart," plus fundere, "to pour"). Physics uses it for particles drifting from high to low concentration through random motion. Sociology borrowed it for the "diffusion of innovations" — how new practices propagate through a population.
noun · /dih-FYOO-zhun/
The gradual spreading of something from where it is concentrated to where it is sparse, driven by nothing more than random movement.
What it names
Why ideas, technologies, and rumors tend to seep outward from a dense pocket and even out over time, without anyone directing the spread.
Example
The new shortcut spread through the office by diffusion — one person used it, their neighbors noticed, and within a month it was everywhere.
Related idea
Diffusion is spreading driven by random mixing that evens things out; a cascade is directed spreading, where each adopter actively triggers the next.