οἶκος

Oikos (οἶκος) meant 'house, household, family estate' in ancient Greek. It was not merely a physical structure but a whole system: the people, the land, the animals, the slaves, the store of grain, the looms, the relationships of obligation that held all of these together. The oikos was the primary unit of Greek society, prior to the polis (city). Before citizens, there were householders. Managing the oikos well was the fundamental practical virtue.

From oikos + nomos (law, management) came oikonomia — household management. Aristotle wrote about oikonomia as the art of maintaining the household so that it sustained itself. The Roman economists translated this as economia; eventually it became economy. For two thousand years after Aristotle, 'economy' still meant something close to its root: the careful management of a bounded system of resources. Adam Smith in 1776 was still thinking of the 'household' when he wrote about the wealth of nations.

From oikos + logos (study, discourse) came ecology — the study of the household, meaning the natural world understood as a system of interrelated inhabitants. Ernst Haeckel coined the term in 1866, consciously invoking the Greek root: nature is a household, and ecology is its management science. The connection Haeckel made is the one Aristotle had implied: economy and ecology are the same discipline applied to different scales. Every economic decision is an ecological one.

Ecumenical, ecumenism — from oikoumenē, 'the inhabited world' — also share the root. The ecumenical movement is the attempt to make the world's Christians into a single household. Diocese, too: from Greek dioikēsis, 'administration of a household,' which the Romans applied to administrative districts. The Roman Empire was, etymologically, a very large house.