What does the inverted comma mean in set theory?

Question

What does the inverted comma mean in set theory?

I have used it, and I don't know what it means.

Here is an example: $(X ∪ X')$

Answer 1

To summarize the comments of others and me, in set theory, $X'$ most commonly denotes the complement of set $X$, i.e., the set of elements not in $X$. For example, working in the set of natural numbers $\mathbb N=\{1,2,3,...\},$ the complement of the set of primes is the set containing composites and $1$. But $X'$ could mean other things in other contexts, and there are other notations used for set complement, so the author should define the symbol before using it.