eu·theria \(ˈ)yü+\noun plural Usage: capitalized Etymology: New Latin, from eu- + Theria : a major division of Mammalia originally coextensive with the subclass Theria but in modern usage an infraclass or other division of Theria comprising the placental mammals as opposed to the Metatheria • eu·the·ri·an\(ˈ)yü|thirēən\adjective or noun