This chapter illuminates what biodiversity is and why it is valuable from an ecological point of view. The term biodiversity, which is a contraction of biological diversity, was coined in its current form around 1985 by the conservation biologist Walter Rosen for the purposes of the National Forum on BioDiversity in Washington, DC that year. Since biodiversity is such a complex, multidimensional and abstract concept, its measurement is far from trivial. Many different measures of biodiversity have been proposed in various contexts: 'At a global scale, there are roughly 40 potential measures being developed for the Convention on Biological Diversity (CBD) and about 26 indicators being considered in the Streaming Biodiversity Indicators in Europe 2010 process'. The biodiversity–ecosystem functioning (BEF) literature reflects a central strand of biodiversity research. The relationship between biodiversity and ecosystem functioning is sometimes called the ecological value of biodiversity.