A petroglyph is an image created by removing part of a rock surface by incising, picking, carving, or abrading, as a form of rock art. Outside North America, scholars often use terms such as "carving", "engraving", or other descriptions of the technique to refer to such images. Petroglyphs are found worldwide, and are often associated with prehistoric peoples. The word comes from the Greek prefix petro-, from πέτρα petra meaning "stone", and γλύφω glýphō meaning "carve", and was originally coined in French as pétroglyphe.
Another form of petroglyph, normally found in literate cultures, a rock relief or rock-cut relief is a relief sculpture carved on "living rock" such as a cliff, rather than a detached piece of stone. While these relief carvings are a category of rock art, sometimes found in conjunction with rock-cut architecture, they tend to be omitted in most works on rock art, which concentrate on engravings and paintings by prehistoric or nonliterate cultures. Some of these reliefs exploit the rock's natural properties to define an image. Rock reliefs have been made in many cultures, especially in the ancient Near East. Rock reliefs are generally fairly large, as they need to be to make an impact in the open air. Most have figures that are larger than life-size.
Stylistically, a culture's rock relief carvings relate to other types of sculpture from the period concerned. Except for Hittite and Persian examples, they are generally discussed as part of the culture's sculptural practice. The vertical relief is most common, but reliefs on essentially horizontal surfaces are also found. The term relief typically excludes relief carvings inside natural or human-made caves, that are common in India. Natural rock formations made into statues or other sculpture in the round, most famously at the Great Sphinx of Giza, are also usually excluded. Reliefs on large boulders left in their natural location, like the Hittite İmamkullu relief, are likely to be included, but smaller boulders described as stele or carved orthostats.
In scholarly texts, a petroglyph is a rock engraving, whereas a petrograph is a rock painting. In common usage, the two words are synonymous. Both types of image belong to the wider and more general category of rock art or parietal art. Petroforms, or patterns and shapes made by many large rocks and boulders over the ground, are also quite different. Inuksuit are also not petroglyphs, they are human-made rock forms found only in the Arctic region.