What is "nestedness"

n. A measure of the order in an ecological system

Nestedness is a measure of structure in an ecological system, usually applied to species-sites systems (describing the distribution of species across locations), or species-species interaction networks (describing the interactions between species, usually as bipartite networks such as hosts-parasites, plants-pollinators, etc.).

A system (usually represented as a matrix) is said to be nested when the elements that have a few items in them (locations with few species, species with few interactions) have a subset of the items of elements with more items. Imagining a series of islands, ordered by their distance from the mainland. If the mainland has all species, the first island has a subset of mainland's species, the second island has a subset of the first island's species, and so forth, then this system is perfectly nested.