Biennial refers to every two years.
Triennial refers to every three years.
Centennial refers to every one hundred years.

What is the similar latin-root adjective meaning every twenty-five (25) years?

Hello.

Septuagesiquintennial

sources:

Illinois Suburban Library Reference: Naming the Anniversaries:
http://www.sls.lib.il.us/reference/por/features/97/annivers.html
Simetric - Time
http://www.simetric.co.uk/si_time.htm

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search strategy:
75th  150th sesquicentennial bicentennial 

I hope this helps.

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Clarification of Answer by juggler-ga on 05 Nov 2004 14:19 PST

Oops, I'm sorry.  I see that you said 25th, not 75th.  Sorry.  I'll
see what I can find.

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Clarification of Answer by juggler-ga on 05 Nov 2004 14:33 PST

Quartocentennial seems to be used.

"Anyone who has spent time seeking the proper Latin term for the
twenty-fifth anniversary (quartocentennial) will appreciate the
"Anniversary Glossary" provided The book's format is visually
appealing"
http://search.barnesandnoble.com/booksearch/isbnInquiry.asp?isbn=0874366755&itm=12

This would make sense as a quarter of a century.

Again, sorry about my earlier misreading.

Perfect.
At first I read your accidental answer and parsed the Latin slowly,
thinking to myself, 'Doesn't that mean 75-year', and then I saw your
clarification.  Well done.

As my 25th wedding anniversary is approaching, I did a bit of research
on this matter a few months ago. I heartily agree with Juggler's
"quartocentennial." This was the first choice given to me by a friend
who is a retired professor of classical languages at the University of
Oklahoma.

Other options: "vigintiquinquennial" and (my favorite) "silver jubilee."

Neat.  I figured 'silver jubilee', intuitively, would be proper, but
that phrase makes me laugh even though it's elegant.  Since a jubilee
technically comes every 50 years (tangent: wouldn't banks just love to
annul debt every 50th year as in the Hebraic tradition?), shouldn't
the 25th be a half-jubilee? :)

In any case, this is for an engraving for a family member who's turning 25.

Grammatoncleric:
Thanks for the tip.

Pink:
Thanks for your comment.