"We define ecoregions as relatively large units of land or water containing a distinct assemblage of natural communities and species, with boundaries that approximate the original extent of natural communities prior to major land-use change. Others have defined ecoregions as areas of ecological potential based on combination's of biophysical parameters such as climate and topography." (source: World Wildlife Fund).
World Wildlife Fund ecologists currently divide the land surface of the Earth into 8 major ecozones containing 867 smaller terrestrial ecoregions (see list[?]). Many consider this classification to be quite decisive, and some propose these as stable borders for bioregional democracy initiatives.
The ecozones are very well-defined, following major continental boundaries, while the ecoregions are subject to more change and controversy. Accordingly the Wikipedia organizes ecology articles under each of the ecozones.