MEGATHERIUM Meaning of MEGATHERIUM in English [ ˌmɛgə'θɪərɪəm ]■ noun ( plural ~s or megatheria ) an extinct giant ground sloth of the Pliocene and Pleistocene epochs.Originmod. L., from Gk mega thērion 'great animal'. Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004