I was looking for a word that means "25 years" in the way that "bicentennial" refers to 200 years.
Wikipedia suggests "Quadranscentennial", but I can't find extensive use of that.
Is there any word for what I'm looking for?
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A 25th anniversary is a silver jubilee or silver anniversary, and a celebration of that milestone would be a Silver Jubilee.
The corresponding 50th anniversary is a golden jubilee, after which usage varies; when celebrating personal achievements it is a diamond jubilee at 60 and platinum at 70, as in the reign of a monarch, but the 75th anniversary of the founding of the Boy Scouts of America, for example, was a diamond jubilee.
If you want to mark a period of time rather than the milestone, then quarter century would do the trick as well as simply saying 25th anniversary or 25 years.
Note the comments entry for quadranscentennial as a term for 25th anniversary in that Wikipedia link:
Probably a modern coined term
Probably correct, in my opinion.
There are a few usages of quadranscentennial as a 25th anniversary event e.g. the Bhutan Quadranscentennial of Apollo 11 commemorative stamp issued in 1994. But I am uncertain if that was a transcription or spelling error, as I could not find any confirmation from an official Bhutan website.
For wedding commemoration, or regency, a term in current use is "Silver Anniversary". Or "Silver Jubilee" in the case of regents. It isn't a Latin-ate term. But it is a commonly understood term.
If semicentennial (semi-, precisely half, + centennial, a period of 100 years) is 50 years, then quarticentennial (quart-, a combining form meaning "a fourth," + centennial) is properly the -ennial word meaning 25 years (and arguably more correct than quadrancentennial, since there is no combining form of quadrant).