Ecology
Ecology is the branch of science that studies the distribution and abundance of living organisms, their habitats, and the interactions between them and their environment — which includes both abiotic (non-living) elements like climate and geology, and biotic ones like other species. The term was coined in 1866 by the German biologist Ernst Haeckel from the Greek oikos meaning "house" and logos meaning "science."
Human ecology is a related but distinct academic discipline which studies humankind, the organized activity of this species, and its environment; it overlaps biological ecology, sociology, and other disciplines.
Outside scientific contexts, the word ecology is often used as synonym of "the environment", i.e. the ensemble of all wild organisms that are living mostly in their ages-old environment and manner, with little human interference; and especially of that part of it that is most important to humanss, for any reason — economical, medical, aesthetical, hedonistic, sentimental, etc.. This sense usually applies when one says that something is good or bad for the "ecology", and in political ecology.
Others may use the word ecology to mean not a science, but a philosophical or even religious system, which implies a specific vision of the universe and specific values and moral imperatives — e.g. that the totality of life is a coherent system, possibly with a purpose; that the extinction of higher species is "bad"; that people should live in harmony with other living beings; and that nature should be protected from human interference. Ecology in this sense is also called environmentalism.