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I've heard $E$ is for entire space, $B$ is for base space, so what is $M$ for?

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No clue, manifold? – Fernando Muro Apr 6 '12 at 10:04
I reckon E is not for "entire space", but for "étalé space" (étalé means "spread out"). – Malte Apr 6 '12 at 11:44
Thom uses notation such as $M(SO(n))$ for the Thom space in his original papers, e.g., Quelques propriétés globales des variétés différentiables. Comment. Math. Helv. 28, (1954). 17–86. So presumably he is responsible for that notation. – Charles Rezk Apr 6 '12 at 12:36
$T$ was already taken for tangent bundle, so $M$ is a pretty good 2nd fiddle. – Ryan Budney Apr 6 '12 at 19:38
It seems that the earliest use is in Thom's paper: – John Klein Apr 6 '12 at 21:48

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