TITLE: Notations that are mnemonic outside of English
QUESTION [51 upvotes]: Some conventional math notations seem arbitrary to English speakers but were mnemonic to non-English speakers who started them. To give a simple example, Z is the symbol for integers because of the German word Zahl(en) 'number(s)'. What are some more examples?

REPLY [2 votes]: Most of the good examples have been mentioned already.

$+$ probably comes from Latin et, "and"
$\tau$ is sometimes used for the golden ratio, from Greek tomos, "section" (same root as atom).
$\sinh$ from Latin sinus hyperbolus (and so on)
$\in$ from Latin est.
$M$ for an arbitrary set from German Menge, crowd.
$\mathrm E[x]$ for expectation is from German Erwartung or French éspérance.
$\aleph$ (the cardinality of the countable numbers) is maybe from Hebrew einsof, infinity, or maybe just because it's the first letter of the Hebrew abjad.
$m$ for slope may be from Latin modulus, "quantity", or French monter, "to climb".
$e$ may be from German Einheit, "unity", because $\ln e = 1$. 
$c$ for the speed of light from Latin celeritas, "swiftness".

If you want to be pedantic, you could point out that there are a large number of mnemonics from other languages which also happen to work in English. For example 

$\mathbb{Q}$, $\mathbb{R}$ from German Quotient, Reel
$\pi$ from Greek perimitron or from Latin peripheriam
$I$ from French intensite de courant
$\%$ comes from Italian per cento, "per hundred"
$x$ is used for variables because it's uncommon in French

You might also count all symbols for the SI prefixes (atto-, yotta-, etc.) except the ones which have become productive in English (micro-, mega-, etc.) But that would be far too pedantic, even for me