The Latin verb diffundere, “to pour in different directions,” gives us the noun diffusion, the verb diffuse, and the adjective diffuse. The noun and the adjective present few difficulties, but the verb is often used ambiguously or incorrectly.
Note: The si in diffusion represents the zh sound. The adjective is pronounced with a soft s sound; the verb is pronounced with a hard s sound:
diffusion noun /di-FJU-zhn/
diffuse adjective /di-FJUS/
diffuse verb /di-FJUZ/
The noun diffusion refers to the action of spreading or dispersing something.
In the context of physics, diffusion is “the permeation of a gas or liquid between the molecules of another fluid placed in contact with it.”
Photographers and painters use the word diffusion to refer to “the process of slightly scattering a portion of the image-forming light to give a pleasing artistic softness to a photograph [or painting].”
Figuratively, diffusion refers to the spreading or scattering of people, customs, or knowledge:
In his Researches into the Early History he ascribes the curious custom of couvade to diffusion, an interpretation that few modern ethnologists would countenance.
Carnegie donated $300,000 to build Washington, D.C.’s oldest library. The building was “dedicated to the diffusion of knowledge.”
The adjective diffuse means “spreading out.” A tree, for example, might have “diffuse branches.” A “diffuse writing style” is wordy. An artist paints a picture in which the light is “diffuse and ethereal.” A population that is not concentrated in one area, but scattered over a region, is diffuse:
Variation in state laws is related to whether the gay and lesbian population is concentrated (where laws permit inequality) or diffuse (where laws promote equality).—The Washington Post.
Like the other words derived from diffundere, the verb diffuse conveys the sense of “scattering or spreading abroad”:
The Japanese intended to diffuse Japanese language and culture throughout the archipelago.
Efforts have been made to diffuse Christianity throughout the world.
When an artist diffuses the light in a painting, the particles of paint that represent light are spread out, producing a softened effect. Perhaps it is this use of diffuse that causes some speakers to use diffuse as if it means “to soften” or “to make less tense.” For example:
While there have been signs that China’s leadership is taking some initiatives intended to diffuse the situation, there are no indications whatsoever that the present tough policy on Tibet will mellow.—Institute of Peace and Conflict Studies.
Or, the writer of the above example may have confused the words diffuse and defuse.
Literally, the verb defuse means “to remove the fuse from an explosive device.” Taking the fuse out of a bomb makes it totally ineffective.
Figuratively, defuse means “to make a situation less tense” or “to make something ineffective.”
If the intended meaning is “make less tense” or “forestall,” ambiguity may be avoided by choosing defuse or some word other than diffuse to express it. Here are some options: