Latin for "almost a plain", a peneplain is the theoretical (fictitious) end stage of the erosion cycle in a moist climate.

A peneplain is an utterly flat landscape where every bit of rock or soil that could have been carried away by rainfall or by a river has been carried away. Long ago, erosion erased all of the land's original surface; now, even the hills and mountains that had remained have been washed away, although an occasional monadnock may provide some relief. Once confined to deep gorges in their youth, and to their floodplains during maturity, senescent rivers now meander freely across what is effectively one immense alluvial plain. Near the ocean, waves have an easy time gnawing away at the shore; we only need to wait for the ocean to eventually consume this sad remnant of a landscape.

The term "peneplain" was coined by William Morris Davis in 1889, to describe the "penultimate denudation" of a landscape. This theory, though instructive, was overly simplistic even before the theory of plate tectonics, revealing Earth's surface to be continually buckling and heaving.

Usually, a potential peneplain is rejeuvenated by tectonic uplift, or some other geological process. Nevertheless, a few areas have been left to erode for so long that they might be given the term, the largest being the North China Plain, bearing two courses of the Huang He separated by a mountain range.

Peneplain-like surfaces can form very close to surfaces much lower and hillier due to the underlying geologic structure (i.e. rocks more resistant to erosion). These are more normally called "equilibrium surfaces" or " pediplanes".