Flow map
A texture whose pixels store direction, used to make another texture appear to stream along a surface — water, lava, mud.
The red and green channels encode a two-dimensional flow direction per point. The shader offsets a water texture along that direction over time, so the current curls around rocks and runs down channels without an expensive fluid simulation. To hide the fact that the offset has to reset, two phases are run offset in time and cross-faded.
The technique was popularised by Valve — Alex Vlachos showed it for Portal 2's water (2011) — and is a textbook example of a cheap fake that looks alive enough. The flow map is often hand-painted or baked from a simulation.