meta·the·ria \ˌ ̷ ̷ ̷ ̷ˈthirēə\noun plural Usage: capitalized Etymology: New Latin, from meta- + -theria 1.: a hypothetical group ancestral to placental mammals postulated to have reached a stage of development equivalent to that of the marsupials 2.in some classifications: a group coextensive with Marsupialia