The Greek word for 'others'.
These days it is used to mean a group of people who share a given culture and live in the same geographical area. It strongly implies that these people see themselves as a group; in modern terms this might be called a nation, although it doesn't need to be defined politically or militarily. It might be expressed as a tribe, a people, a caste, or an ethnic group, among other labels.
Ethnos is the root from which we get words like ethnic, ethnology, ethnographic, and ethnocentrism.