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From this textbook:

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And this textbook:

enter image description here

It looks like a squiggly E (\mathcal{E}) or perhaps it is the Greek letter xi (ξ). But I am not sure.

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    You should probably ask this on the tex.stackexchange site... – 5xum Mar 15 '16 at 07:13
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    It’s simply an upper-case script E. \mathscr{E} ($\mathscr{E}$) comes a bit closer to it than \mathcal{E} ($\mathcal{E}$), though neither is especially close to that particular font. – Brian M. Scott Mar 15 '16 at 07:18
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    @5xum: Why? It’s a straightforward question about the meaning of a piece mathematical notation. – Brian M. Scott Mar 15 '16 at 07:19
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    Actually the OP already knows the meaning (the universal set), but wants to know which character it is. – Robert Israel Mar 15 '16 at 07:20
  • @BrianM.Scott I don't see this question as asking "what is the meaning of this symbol", especially since both use cases clearly state that the symbol means "the universal set". The question is really about what symbol this is, not what it means, and therefore a better fit for tex.SE – 5xum Mar 15 '16 at 07:25
  • @5xum: Sorry: that was a slip of the mind: I meant to write identity, not meaning — and I consider that entirely appropriate here. – Brian M. Scott Mar 15 '16 at 07:27
  • @Robert: Yes; see my comment response to 5xum just now. – Brian M. Scott Mar 15 '16 at 07:27

1 Answers1

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It looks like a capital E in a script style font. It is not $\Xi,\xi$.

enter image description here

More details can be found on the tex site here.

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