Other forms: diffused, diffusing, diffuses
To diffuse is to spread out in every direction until the concentration is gone — light through frosted glass, scent through a room, ink through water. The picture is always of something becoming thinner as it spreads.
The adjective is the same spelling and a different sound: the verb ends in a z sound, the adjective in an s. Diffuse light is soft and shadowless; a diffuse argument is one that wanders. And there is a trap worth naming: diffuse is not defuse. To defuse is to take the fuse out of a bomb, and so to calm a tense situation — defuse the row, not diffuse it. The two are confused constantly in print, and the difference is a whole letter and a whole meaning.
1verbdiffuseTo spread out in all directions and become less concentrated.
“The shade diffuses the light across the whole room.”
2adjectivesoftSpread out and soft rather than sharp or concentrated.
“Diffuse light is kinder to photograph in.”
3adjectivewanderingOf writing or speech, wandering and not to the point.
“The report is diffuse and about twice as long as it needs to be.”
diffuse is a way of spreading.
Latin diffundere means to pour in different directions — dis-, apart, plus fundere, to pour, the same verb behind fuse, confuse and refund. Everything the word does is still pouring outwards until nothing is concentrated any more.
Proto-Indo-European
*ǵʰewd-
Latin
diffūsus
Middle French
diffuser
English, today
diffuse
From Middle French diffuser, from Latin diffūsus, past participle of diffundo, from dis- + fundo.
Etymology from Wiktionary, CC BY-SA