A repetitive geometric pattern, often used as background in Gothic illumination.
From the French diapré ('variegated'), a diaper pattern is a repetitive geometric pattern. Although used as early as the eleventh century, it often acted as a background in Gothic illumination. Some artists even seem to have specialized in diaper grounds.
Michelle Brown. Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).