The World Wildlife Federation defines an ecoregion as "relatively large units of land or water containing a distinct assemblage of natural communities sharing a large majority of species, dynamics, and environmental conditions" 1. An ecoregion is a biome broken down even further. There are 867 ecoregions comprising the world's terrestrial and marine ecology. Nearly half of the world's terrestrial ecoregions (391) are within the hotspots.
1 World Wildlife Fund, "Terrestrial Ecoregions of the World," http://www.worldwildlife.org/publications/terrestrial-ecoregions-of-the-world (accessed September 6, 2016).
D. M. Olson, et al., "Terrestrial ecoregions of the world: a new map of life on Earth," Bioscience 51, no. 11 (2001): 933-938. Available at http://www.worldwildlife.org/publications/terrestrial-ecoregions-of-the-world.
Critical Ecosystem Partnership Fund, "The Biodiversity Hotspots," http://www.cepf.net/resources/hotspots/pages/default.aspx (accessed July 1, 2014). Data made available under the Creative Commons BY-SA 4.0 License: https://creativecommons.org/licenses/by-sa/4.0/legalcode.