A wavy pattern of lines or colored bands that can appear in a photo when a fine, regular pattern in the subject, such as fabric, a screen or a building facade, overlaps with the regular grid of pixels on a digital sensor. The two patterns interfere with each other and create a third pattern that is not really there.
Many cameras have an optical low-pass, or anti-aliasing, filter in front of the sensor to reduce moiré. Some cameras leave this filter out to gain sharpness, so moiré is more likely with them. Changing the distance, angle or focus usually helps.
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