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While browsing through several pages of nlab(mainly on n-Categories), I encountered the notion "foo" several times. However, there seems to be article on nlab about this notion. Is this some kind of category theorist slang? Please explain to me what this term means.

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I think it is only used as a "variable"... Do you have an example? –  M Turgeon Jun 20 '13 at 14:38
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To enhance M Turgeon's anwser : en.wikipedia.org/wiki/Foobar. –  Pece Jun 20 '13 at 14:43
    
    
It's a reasonable guess that this originates in military slang as an acronym, en.wikipedia.org/wiki/Snafu and en.wikipedia.org/wiki/List_of_military_slang_terms#SNAFU –  Will Jagy Jun 20 '13 at 18:56
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@Will: I think you actually want en.wikipedia.org/wiki/List_of_military_slang_terms#FUBAR . –  Qiaochu Yuan Jun 20 '13 at 19:45
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up vote 12 down vote accepted

It's slang, which I've mostly seen used in the context of computing rather than category theory; foo is just a placeholder for something else, as is bar. A logician I know likes talking about widgets and wombats $-$ it all serves the same purpose.

For example, you might say "an irreducible foo is a foo with no proper sub-foos".

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Irreducible is a nice property for illustrating foos: +1 –  Georges Elencwajg Jun 20 '13 at 14:46
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