Depiction of erotic behaviour intended to cause sexual excitement. The word originally signified any work of art or literature depicting the life of prostitutes. Though pornography is clearly ancient in origin, its early history is obscure because it was customarily not thought worthy of transmission or preservation. Nevertheless, in the artwork of many historic societies, including ancient India, ancient Greece, and Rome, erotic imagery was commonplace and often appeared in religious contexts. The Art of Love, by Ovid, is a treatise on seduction and sensual arousal. The invention of printing led to the production of ambitious works of pornographic writing intended to entertain as well as to arouse. In 18th-century Europe, pornography became a vehicle for social and political protest through its depiction of the misdeeds of royalty and other aristocrats, as well as those of clerics, a traditional target. The development of photography and motion pictures in the 19th and 20th centuries contributed greatly to the proliferation of pornography, as did the advent of the Internet in the late 20th century. During the 20th century, restrictions on pornography were relaxed throughout much of Europe and North America, though regulations remained strict in Asia, the Middle East, and Africa. Child pornography is almost universally prohibited.