Biodiversity or biological diversity is the diversity of life. There are a number of definitions and a measure of biodiversity. Biodiversity has no single standard definition. The most basic definition is "variation of life at all level of biological organization". Another definition holds that biodiversity is a measure of the relative diversity among organisms present in diverse ecosystems. "Diversity" in this definition includes diversity within a species and among species, and comparative diversity among ecosystems.
A third definition that is often used by ecologists is the "totality of genes, species, and ecosystems of a region". An advantage of this description is that it seems to describe most circumstances and present a unified view of the traditional three levels at which biodiversity has been identified.
The 1992 United Nations Earth Summit in Rio de Janeiro defined "biodiversity" as "the changeability among living organisms from all sources, including, 'inter alia', terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are part: this includes diversity within species, between species and of ecosystems".