The example I am thinking of is GNU, which stands for "GNU's not unix".
It may be that there are no other examples. And maybe that's the whole joke. I was just wondering.
It's usually called a recursive acronym. It draws, by its very nature (and geekiness), a certain fondness from computer hobbyists and is thus heavily used in naming software packages, programming concepts or algorithms. There are, however, some non-technical examples (drawn from the Wikipedia page linked above):
Also of note: “GNU Hurd” is a mutually recursive acronym, where “Hurd” stands for “Hird of Unix-Replacing Daemons”, and “Hird” stands for “Hurd of Interfaces Representing Depth”.
They're called "recursive acronyms":
For more examples, read DR Hofstadter's "Gödel, Escher, Bach: an Eternal Golden Braid" - that's where I first heard of them, and many other wonders of language/math/science/art/computers/music as well.
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