How to define a mountain is controversial. This controversy results in part from the fact that different mountain definitions have been proposed for different contexts of use and even for different countries. Accordingly, a definition tailored for applications related to life conditions for humans, plants, and animals may not serve the purpose of other applications (e.g., hydrology and climatology).
Regardless of the context of use, defining a mountain requires to make a distinction between elevation (and its implications such as reduced barometric pressure and atmospheric temperature), and steepness as a measure of inclination and gravitational forcing.
In 2011, GMBA proposed a definition of mountains that constrains mountains by ruggedness of terrain only, irrespective of elevation. Ruggedness is defined as the maximal elevational difference among 3 x 3 = 9 grid points of 30" in 2.5' pixels and is calculated based on the digital elevation model used by WorldClim (Hijmans et al. 2005). For a 2.5' pixel to be defined as rugged (i.e., 'mountainous'), the difference between the lowest and highest of the 9 points must exceed 200 m.
The GMBA definition differs from the widely-used definition proposed by Kapos et al. (2000), which constrains mountains by a combination of elevation and ruggedness. Accordingly, based on the GMBA definition mountains cover 12.3 % of the Earth's surface outside of Antarctica and are home to 511 million "mountain people", whereas based on the alternative definition by Kapos et al. mountains cover approximately 22% of the Earth's surface and host 1.27 billion people.
Detailed information about the mountain definition is available in Körner et al (2011) (PDF, 2.0 MB). The ruggedness data are available in various format for unrestricted download and usage. The ruggedness layer displayed in the GMBA Mountain Portal and used as baseline for the GMBA mountain inventory is based on the GMBA mountain definition.
1When using the data please cite Körner et al., 2011 and the dataset DOI: 10.7892/boris.83486