Moire (pronounced "more-RAY") is a optical effect caused by two intersecting patterns. It is common when working with printing screens, but can also manifest with finely-patterned image elements. The classic example is the lines that appear when scanning a half-tone image.

It is usually associated with scanning (http://www.creativepro.com/story/feature/10744-1.html), but can also pop up when applying manipulations to finely-patterned subjects.