What does diffusion mean?
Diffusion is the process by which something spreads out from where it originates until it is dispersed widely. In its most literal sense, used in physics and chemistry, diffusion describes how molecules move spontaneously from regions of higher concentration to lower concentration — the reason a drop of ink slowly colors an entire glass of water without any stirring. Beyond the laboratory, the word is widely applied to the transmission of ideas, technologies, customs, and innovations through societies, as in 'the diffusion of printing' or 'the diffusion of democratic ideals.' It can also describe the scattering of light, which is why photographers speak of diffused lighting that softens shadows. Derived from the Latin verb diffundere, meaning 'to pour out or spread abroad,' diffusion carries a connotation of gradual, often natural spread rather than forceful imposition. It is a precise yet versatile term, equally at home in a chemistry textbook and a work of social history.
The process by which particles of one substance spread and intermingle with another due to their inherent molecular motion.
"Oxygen passes into the bloodstream by diffusion across the walls of the lungs."
This is the dominant sense in scientific registers.
The wide dissemination or dispersal of ideas, knowledge, customs, or technologies among people.
"The diffusion of literacy following the printing press reshaped European society."
Common in sociology, anthropology, and history.
The scattering of light or other radiation in many directions, producing softness or reduced glare.
"A sheer curtain provides gentle diffusion of the afternoon sunlight."
The plural is rare because diffusion usually names an uncountable process. When pluralized, it typically refers to distinct instances or types of diffusion in scholarly writing.
"The paper compares the diffusions of two distinct technologies across different markets."
Drop a tea bag in hot water and you're watching the same physics that spread the printing press, jazz, and the internet — no stirring required.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of diffusion
Diffusion comes from the Latin diffusio, formed on the verb diffundere, meaning 'to pour forth, scatter, or spread abroad,' from dis- ('apart, in different directions') plus fundere ('to pour'). The word entered English in the late Middle English period, initially carrying the general sense of spreading or pouring out, before acquiring its specialized physical-science sense as molecular theory developed in later centuries. Its Latin root also gives us related borrowings such as effusion, transfusion, and infusion, all built on fundere, and the adjective diffuse remains its direct sibling.
Related word forms
How diffusion is actually used
Diffusion is a formal to neutral term, most at home in scientific writing (physics, chemistry, biology) and academic discussions of cultural change. In physics contexts it often appears with technical modifiers such as 'diffusion rate' or 'diffusion coefficient.' The figurative use — the diffusion of ideas or influence — is well established and carries no strong connotation, though it can suggest slow, organic spread rather than deliberate distribution.
Easily confused with diffusion
Infusion means pouring or steeping a substance into something else (or introducing one thing into another), whereas diffusion means spreading outward so that something becomes dispersed.
Effusion refers to the escape of a fluid or gas through a small opening, or an outpouring of emotion, while diffusion refers to gradual spreading and intermingling throughout a space or population.