Meaning of # in descriptions of languages This is a very simple question, but I cannot find the answer anywhere, mostly because I don't know how else to ask about what I'm looking for. I have a homework assignment that has to do with pumping lemma, which asks me to use the pumping lemma to prove that the language given is not regular.

{x₁#x₂#x₃|x₁, x₂, x₃ ∈ {a, b}* and x₂ = x₃ᴿ}.

My only question is this — what does the # mean? My teacher is currently unavailable, so I'm unable to ask him.

If someone has a link to a list of notations and their descriptions, I'd very much appreciate it, but if you're only able to tell me what # means, that's fine as well.
It's just a symbol in the alphabet, so the alphabet is $\{a,b,\#\}$ and the language is the set of strings that:


contain exactly two $\#$'s and
the string after the second $\#$ is the reverse of the string between the two $\#$'s.