#9: Quinquennium

Definition:

: a period of five years

Example:

"After the age of 50, that is, when the working period of life is over, the death-rates in asylums rise from quinquennium to quinquennium by a considerable but irregular progress." – Journal of the Institute of Actuaries, 1890

About the Word:

It's somewhat odd that everyone knows the word for a span of ten years (decade) and certainly we all know the word for a year's time (year), yet so few of us know the word for a span of five years.

But it is not as odd as it seems; the English language is awash in interesting words for time that no one ever uses. In addition to quinquennium we have biennium (a period of two years), duodecennial (occurring once in 12 years), quadrennium (a period of four years), septennium (a period of seven years), and sexennial (continuing or lasting six years).