A more complete definition describes them as “Areas of relatively homogeneous species composition, clearly distinct from adjacent systems” dominated by “a small number of ecosystems and/or a distinct suite of oceanographic or topographic features”. Ecologically they “are strongly cohesive units, sufficiently large to encompass ecological or life history processes for most sedentary species.” 
The global classification system Marine Ecoregions of the World—MEOW was devised by an international team, including major conservation organizations, academic institutions and intergovernmental organizations.  The system covers coastal and continental shelf waters of the world, and does not include deep ocean waters. The MEOW system integrated the biogeographic regionalization systems in use at national or continental scale, like Australia's Integrated Marine and Coastal Regionalisation of Australia and the Nature Conservancy’s system in the Americas, although it often uses different names for the subdivisions. 
This system has a strong biogeographic basis, but was designed to aid in conservation activities for marine ecosystems. Its subdivisions include both the seafloor ( benthic) and shelf pelagic ( neritic) biotas of each marine region. 
The digital ecoregions layer is available for download as an ArcGIS Shapefile. 
The provinces in turn, are grouped into 12 major realms. The latter are considered analogous to the eight terrestrial realms, represent large regions of the ocean basins:
- Temperate Northern Atlantic
- Temperate Northern Pacific
- Tropical Atlantic
- Western Indo-Pacific
- Central Indo-Pacific
- Eastern Indo-Pacific
- Tropical Eastern Pacific
- Temperate South America
- Temperate Southern Africa
- Temperate Australasia
- Southern Ocean
Other classifications of marine ecoregions or equivalent areas have been widely developed at national and regional levels, as well as a small number of global schemes.
Each of these systems, along with numerous regional biogeographic classifications, was used to inform the MEOW system. The WWF Global 200 work also identifies a number of major habitat types that correspond to the terrestrial biomes: polar, temperate shelves and seas, temperate upwelling, tropical upwelling, tropical coral, pelagic (trades and westerlies), abyssal, and hadal (ocean trench).
- Briggs Coastal Provinces
One of the most comprehensive early classifications was the system of 53 coastal provinces developed by Briggs in 1974.  The near-global system of 64 large marine ecosystems has a partial biogeographic basis.
- WWF Global 200
- Spalding, Mark D., Helen E. Fox, Gerald R. Allen, Nick Davidson et al. "Marine Ecoregions of the World: A Bioregionalization of Coastal and Shelf Areas". Bioscience Vol. 57 No. 7, July/August 2007, pp. 573–583. 
- "MEOW digital ecoregions layer: ArcGIS Shapefile download link". Archived from the original on 2012-07-19. Retrieved 2010-12-03.
- Briggs JC (1974) Marine Zoogeography. McGraw-Hill, New York, USA
- Olson DM, Dinerstein E. 2002; "The Global 200: priority ecoregions for conservation"; Annals of the Missouri Botanical Garden 89: 199-224
|Wikimedia Commons has media related to Marine ecosystems.|