Keeping time – from a second to a millennium – is an essential ingredient in our lives and the way we measure them.

We have special words for describing these moments in time.  Since a new millennium began not too many years ago, we are all familiar with that term, as we are with centuries (with centennial or centenary as adjectives), and perennial – something which occurs annually.  Here are a few that we don’t use as often:

Biennial – occurring every two years (as opposed to biannual , which is twice per year)
Decennial – occurring once a decade (also denary is used at times).
Vicennial – occurring every twenty years
Quadranscentennial – a 25th anniversary year
Triennial – occurring every thirty years
Quadrennial – occurring every forty years
Quintennial – occurring every fifty years (in ancient times, it was called a jubilee year ).
Sesquicentennial – occurring every one hundred and fifty years
Dodransbicentennial – occurring every 175 years
Bicentennial – a two hundredth anniversary
Sestercentennial – every 250 years or a quarter of a millennium (also semiquintennial can be used)
Tercentennial – every 300 years (also called tricentennial at times)
Quadricentennial – every 400 years
Quincentennial – every 500 years, or a half of a millennium
Sexacentennial – every 600 years
Septuacentennial – every 700 years
Octocentennial – an 800 th anniversary year
Nonagintennial – a 900 th anniversary year
Sesquimillennial – occurring every 1500 years
Bimillennial – every 2000 years
Trimillennial – every 3000 years
Decamillennial – occurring every 10,000 years

We could keep going into the billions and trillions, but usually don’t need to use those terms.  We never know, though, when we might need to use the above milestones.  On July 4, 2026, we will celebrate the sestercentennial year of the birth of the United States.  In 2013, Gettysburg celebrated its sesquicentennial year of the battle here.  In 2022, this magazine will complete its quadranscentennial year of publication.  On February 22, 2032 – just a decade away – we will pass the tercentennial anniversary of the birth of our first U.S. President, George Washington. 

We have perfect words for milestone years – it’s a good thing to know them.

Pass them on!