Ecology; the natural environment.
[Greek oikos, house.]
Ecology deals with the relations of organisms to one another and to their physical surroundings. Several terms in scientific use the prefix in this strict sense: ecosystem, a biological community of interacting organisms and their physical environment; ecosphere, the biosphere of the earth or other planet, especially when the interaction between the living and non-living components is emphasized; ecotype, a distinct form or race of a plant or animal species occupying a particular habitat.
The prefix is more common in a broader sense that refers to the natural environment, especially something that aids its preservation; in this sense it is intimately linked to green issues and the environmentalist movement. A product that is eco-friendly does not harm the environment; ecotourism tourism directed towards exotic, often threatened, natural environments, especially to support conservation efforts and observe wildlife; ecoterrorism is violence carried out to further environmentalist ends; ecofeminism is a philosophical and political theory and movement which parallels and combines environmental concerns with feminist ones.