An ecoregion (ecological region), sometimes called a bioregion, is an ecologically and geographically defined area smaller than a "realm" or "ecozone". Ecoregions cover relatively large areas of land or water, and contain characteristic, geographically distinct assemblages of natural communities and species. The biodiversity of flora, fauna and ecosystems that characterise an ecoregion tends to be distinct from that of other ecoregions.