Wiktionary
n. 1 Regions of a fluid having different refractive index because of their different density 2 (context geology English) Irregular streaks of different composition in some igneous rocks
Wikipedia
Schlieren (from German; singular "Schliere", meaning "streak") are optical inhomogeneities in transparent material not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation. This light deviation can produce localized brightening, darkening, or even color changes in an image, depending on which way the ray deviates.
Usage examples of "schlieren".
I have some old schlieren photos in my office showing you how terrible.
And he showed Mott a high-speed schlieren photograph of an unsatisfactory model perched atop a narrow steel pylon, more than a hundred minute wires leading from a hundred sensors mounted on various interior parts of the plane.
They would skirt the center of Zurich and head toward Kloten, turning right at the Schlieren fork and follow the central road toward Bulach.
They were patterns of air currents, flowing volumes of air made visible by color-lit fogs of vapor, fractally rich with eddies and schlieren.