A binary operator representing set difference or relative complement between two sets in set theory.
Examples
Set difference with small sets
\{1, 2, 3, 4\} \smallsetminus \{2, 4\} = \{1, 3\}
Set difference with number sets
\mathbb{R} \smallsetminus \mathbb{Q} \text{ represents all irrational numbers}
Multiple set differences
A \smallsetminus (B \smallsetminus C) = (A \smallsetminus B) \cup (A \cap C)