I've heard that several category and topos theorists, first and foremost Johnstone (see the comments to this question) adamantly insist on 'toposes' as the plural of 'topos'. I was wondering whether there is some reason beyond pure preference here and what the history of these variants is.

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    $\begingroup$ Why (almost) all mathematicians adamantly insist with using the anglicized names of some Greek letters (e.g. "mu", "nu")? Why they adamantly insist in pronuncing the Greek letter $\xi$ as /ˈsaɪ/? Why (almost) all mathematicians adamantly insist in (quite ridiculously!) pronouncing Latin words (e.g. "moduli", "vice versa", ...) according to English pronounciacion rules? Maybe it's just because English, as any other living language, absorbs words from other cultures and languages. And English happens to be the present international language of science. So... $\endgroup$ – Qfwfq Sep 14 '15 at 15:55
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    $\begingroup$ I suggest the OP asks on one of the SE sites about English usage. Johnstone himself asks whether people carry hot beverages during hiking in thermoi. $\endgroup$ – Yemon Choi Sep 14 '15 at 16:31
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    $\begingroup$ @Qfwfq: "Mu" is not an anglicized form. In ancient Greek, the name of the letter was μῦ, and the original pronunciation of the letter υ was "oo." In British English, people usually pronounce the name of the letter as "moo," which is the way it would have been pronounced in ancient Greek. Maybe you're thinking of the modern Greek name and pronunciation, which are different. Or maybe you're thinking of the American pronunciation "myoo?" $\endgroup$ – Ben Crowell Sep 14 '15 at 21:31
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    $\begingroup$ Ben, it depends what you call ancient Greek. In archaic Greek, say like 8th century BC, the letter $\upsilon$ (upsilon) was indeed pronounced oo. When Latin people borrowed their alphabets to Greeks through Etruscans, there was no difficulty with this letter, which has drawn similarly and corresponded to the same sound. But later, in classical Greek (about fifth-fourth century BC), it was pronounced u as in French. Even later, in the koine (say second century BC) it was pronounced i, which is the pronunciation it sill has in modern Greek. The Romans then reborrowed the letter u to make the Y. $\endgroup$ – Joël Sep 15 '15 at 2:00
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    $\begingroup$ Because they are ignorami. $\endgroup$ – Georges Elencwajg Sep 15 '15 at 7:21