\setminusThe \setminus operator is used to denote the set difference between two sets. In set theory, if A and B are sets, then A \setminus B represents the set of elements that are in A but not in B.
Set difference between sets A and B
A \setminus B = \{ x \in A \mid x \notin B \}The set of all irrational numbers, which is the set of real numbers excluding the rational numbers
\mathbb{R} \setminus \mathbb{Q}The set S excluding the element a
S \setminus \{ a \}