Domain warping
Distorting a noise function's input coordinates with another noise function, so flat gradients fold into swirling, organic patterns like marble or cloud.
Plain Perlin or simplex noise looks smooth but lifeless. Domain warping feeds the point through noise before sampling it: instead of noise(p) you compute noise(p + noise(p)), ideally in several layers. The pattern is pushed and pulled locally and starts to resemble the veining in marble, tangled clouds or eroded terrain.
The technique was popularised by Inigo Quilez through his Shadertoy sketches and articles, and is a staple of procedural texturing and landscape generation. It is often stacked on top of fractal noise for still richer structure.