Fractal Brownian motion
Layers of noise stacked from large and soft to small and fine, for clouds, rock and hills.
See it in Noise & texture. Set the noise type to fBm, then change Octaves, Lacunarity and Persistence.
In plain words
Fractal Brownian motion, or fBm, is a recipe for natural-looking noise. Start with broad, soft noise. Add a layer twice as fine and half as strong, then another, and so on. Big shapes and small details now sit together, the way they do in real clouds and mountains. Most terrain and cloud tools start from fBm.