A moiré pattern is an interference pattern created when two sets of overlapping grids are placed together at an angle to each other. They are quite common in scanned images due to the dot patterns used in printing, when the image interferes with itself on either side of the scanner's glass surface.
The term originates with weavers and comes from the word mohair, a cloth made from the fine hair of an Angora goat. They noticed that the fabric would form odd patterns when woven with different tensions on the hair.
