This one is pretty well-known: the notation $e$ for the identity of a group comes from the German word *Einheit*, meaning *unit*.

I'd be willing to bet that the notation $G$ for a group also comes from German... but we don't notice, because the German word for *group* is *Gruppe*!

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[Here's a fun one][1]: the notation $Z$ for a topological quantum field theory comes indirectly from the notation $Z$ for a partition function in statistical mechanics, which comes from the German word *Zustandssumme*, meaning *state sum*. I said "indirectly" because partition function in quantum field theory isn't a statistical-mechanical partition function... it just looks like one after you Wick rotate! (Then again, maybe there's a deeper sense in which the QFT partition function really *is* a statistical-mechanical partition function. Does anybody know?)


  [1]: http://ncatlab.org/nlab/show/partition+function