A diffuse object or situation spreads out and becomes unclear.
You might hear someone say that light from a lamp diffuses across the room. It means the light isn't shining brightly in one spot but instead fills up the space evenly. In this case, "diffuse" describes something that's spreading out, not concentrated in one area. If you're trying to solve a mystery and the clues start to diffuse, it means they get harder to follow and become unclear.
To spread over or through as in air, water, or other matter, especially by fluid motion or passive means.
