From Wikipedia:

In physics, a moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes. Comes from the French word for a type of fabric–generally silk–that has a rippled appearance.

Par example:

Check www.mathematik.com, to see Moiré in motion.

Or:

Computer-rendered 7-slit Optical Interference Pattern:

Physics, kids!