To reestablish the presumed structure, productivity and species diversity of the forest originally present on the site (in time, the ecological processes and functions of the restored forest will closely catch those of the original forest)
WWF/IUCN - FLR, 2000

The process of assisting the recovery of an ecosystem that has been degraded, damage, or destroyed
Society for Ecological Restoration, 2000

The process of intentionally altering a site to establish a defined, indigenous, historic ecosystem. The goal of this process is to emulate the structure, function, diversity, and dynamics of the specified ecosystem.
USDA-NRCS, ????

The process of returning an ecosystem as closely as possible to predisturbance conditions and function. Implicit in this definition is that ecosystems are naturally dynamic; it is therefore not possible to recreate a system exactly. Te restoration process reestablishes the general structure, function, and dynamic but self-sustaining behavior of the ecosystem. Restoration differs from rehabilitation in that restoration is a holistic process not achieved through the isolated manipulation of individual elements. While restoration aims to return an ecosystem to a former natural condition, rehabilitation implies putting the landscape to a new or altered use to serve a particular human purposes
Willamette Restoration Initiatives, 1999