The variety of life across all levels of organization from genetic diversity within populations, to species, which have to be regarded as the pivotal unit of classification, to ecosystems.
the variety and variability among living organisms and the ecosystems in which they occur. Biodiversity includes the numbers of different items and their relative frequencies; these items are organized at many levels, ranging from complete ecosystems to the biochemical structures that are the molecular basis of heredity. Thus, biodiversity encompasses expressions of the relative abundances of different ecosystems, species, and genes.
The variety of different species, the genetic variability of each species, and the variety of different ecosystems that they form.