TRIENNIUM Meaning of TRIENNIUM in English [ trʌɪ'ɛnɪəm ]■ noun ( plural triennia -nɪə or ~s ) a period of three years.OriginC19: from L., from tri- 'three' + annum 'year'. Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004