triennium
Concise Oxford English Dictionary © 2008 Oxford University Press:
triennium /trʌɪˈɛnɪəm/
▶noun (pl. triennia /-nɪə/ or trienniums) a period of three years.
– origin C19: from L., from tri- ‘three’ + annum ‘year’.
'triennium' also found in these Oxford entries:

