Moiré pattern

A moiré pattern is the interference pattern created when two geometrically regular patterns are superimposed. The combination or synthesis of the overlayed patterns creates the moiré interference pattern.

Moiré patterns often appear when halftone graphics (like those found in print newspapers and magazines) are electronically scanned.

Increasing moire patterns are a sign that a natural inhabitant of the Virtu is about to pass on in Roger Zelazny's Donnerjack.

This Moire effect is very handy when you don't have whimsical panties/tights: just wear 2 tights (same brand, black ones 20 or 30 denier for the best effect) on top of each other and the pattern changes while moving your legs.
I don't think Moire invented it with that purpose.

I tried to explain this Moire pattern to a collegue, but I didn't want him to fixate his looks on my legs constantly, so I'd showed him http://www.sandlotscience.com/Moire/Moire_frm.htm , wich has some interactive examples.