Ecology (from Greek: οἶκος, oikos, "house"; -λογία, -logia, "study of") is the interdisciplinary scientific study of the distribution and abundance of organisms and their interactions with their environment. The environment of an organism includes all external factors, including abiotic ones such as climate and geology, and biotic factors, including members of the same species (conspecifics) and other species that share a habitat. If the general life science of biology is viewed as a hierarchy of levels of organization, from molecular processes, to cells, tissues and organs, and finally to the individual, the population and the ecosystem, then the study of the latter three levels belongs within the purview of ecology.

Examples of objects of ecological study include: Population processes, including reproductive behavior, mortality, bioenergetics and migrations, interspecific interactions such as predation, competition, parasitism and mutualism, plant and animal community structures and their function and resilience, and biogeochemical cycling. Because of its vast scope, ecological science is often closely related to other disciplines. Thus, molecular ecology addresses ecological questions using tools from genetics, paleoecology uses tools from archeology, and theoretical ecologists use often highly complex mathematical models to explore how ecosystems and their elements function.

Aside from pure scientific inquiry, ecology is also a highly applied science. Much of natural resource management, such as forestry, fisheries, wildlife management and habitat conservation is directly related to ecological sciences and many problems in agriculture, urban development and public health are informed by ecological considerations.

The term "ecology" has also been appropriated for philosophical ideologies like social ecology and deep ecology and is sometimes used as a synonym for the natural environment or environmentalism. Likewise "ecological" is often taken in the sense of environmentally friendly.