biennium
bi.en.ni.um \bi--'en-e--*m\ \-e--*\ n or bienniums or bi.en.nia [L, fr. bi- + annus year - more at ANNUAL] pl : a period of two years
Webster's English Dictionarybi.en.ni.um \bi--'en-e--*m\ \-e--*\ n or bienniums or bi.en.nia [L, fr. bi- + annus year - more at ANNUAL] pl : a period of two years