What does e pluribus unum mean in English?

"E pluribus unum" means "One out of many".

On a side note:

Ladies and gentlemen, please keep such discussion in the "Discussion" tab of questions. This is inappropriate for the actual answer, and indeed you have removed all information pertinent to the question in your search for the correct instances to use "ex" rather than "e".

I have retained the text below because I find it interesting linguistically, and I cannot move it to the Discussion area without it all being under my username.

All right! How do you explain the forms:

ex cathedra

ex consensu

ex nihilo, nihil fit

ex professo

ex voto donatum

even, de ex de, from which the Spanish preposition "desde" stems.

I have never seen those expressions with "e"

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Do you mean why they use "ex" when there's a consonant in front? "e" before a vowel, and "ex" before a consonant is more of a guideline than a hard and fast rule. Even in classical Latin, you'll see "ex" in front of consonants. But you'll never see "e" before a vowel. I should've been clearer, and I'm sorry about that.

As far as I know, except for "ex nihilo...," those are from Ecclesiastical Latin, and it's possible that "e" fell out of use. But that's just a guess, I don't know ("non ex professo" :-))