thumb|300px|Aerial view of the almost flat and drowned peneplain at [[Belcher Islands, Hudson Bay, Canada, cutting across numerous geologic folds.]]
In geomorphology and geology, a peneplain is a low-relief plain formed by protracted erosion. This is the definition in the broadest of terms, albeit with frequency the usage of peneplain is meant to imply the representation of a near-final (or penultimate) stage of fluvial erosion during times of extended tectonic stability. but Davis and other researchers have also used the term in a purely descriptive manner without any theory or particular genesis attached.
Discussion
thumb|390px|Sketch of a hypothetical peneplain formation after an [[orogeny.]]
The existence of some peneplains, and peneplanation as a process in nature, is not without controversy, due to a lack of contemporary examples and uncertainty in identifying relic examples. By some definitions, peneplains grade down to a base level represented by sea level, yet in other definitions such a condition is ignored.
While peneplains are usually assumed to form near sea level it has also been posited that peneplains can form at height if extensive sedimentation raises the local base level sufficiently or if river networks are continuously obstructed by tectonic deformation. The peneplains of the Pyrenees and Tibetan Plateau may exemplify these two cases respectively.
At the grand-scale peneplains are characterized by appearing to be sculpted in rock with disregard of rock structure and lithology, but in detail, their shape is structurally controlled, for example, drainage divides in peneplain can follow more resistant rock. In the view of Davis large streams do become insensitive to lithology and structure, which they were not during the valley phase of erosion cycle. This may explain the existence of superimposed streams. and glacial erosion among processes that can contribute in shaping peneplains. Exhumed peneplains are those that are re-exposed after having been buried in sediments.
Pediplains
The peneplain concept is often juxtaposed to that of pediplain. However authors like Karna Lidmar-Bergström classify pediplains as a type of peneplain.
:A peneplain in the Davisian sense, resulting from slope reduction and downwearing, does not exist in nature. It should be redefined as "an imaginary landform."
According to King the difference between pediplains and Davis’ peneplains is in the history and processes behind their formation, and less so in the final shape. A difference in form that may be present is that of residual hills, which in Davis’ peneplains are to have gentle slopes, while in pediplains they ought to have the same steepness as the slopes in the early stages of erosion leading to pediplanation.]]
Peneplains that are detached from their base level are identified by either hosting an accumulation of sediments that buries it or by being in an uplifted position. Burial preserves the peneplain. Any exposed peneplain detached from its baselevel can be considered a paleosurface or paleoplain. Uplift of a peneplain commonly results in renewed erosion. As Davis put it in 1885:
:"the decrepit surface must wait either until extinguished by submergence below the sea, or regenerated by elevation into a new cycle of life"
Uplifted peneplains can be preserved as fossil landforms in conditions of extreme aridity or under non-eroding cold-based glacier ice. In the Fennoscandian Shield average glacier erosion during the Quaternary amounts to tens of meters, albeit this was not evenly distributed.