A relational symbol representing "such that" in set-builder notation or conditional probability statements.
Examples
Set builder notation (such that)
\{x \in \mathbb{R} \mid x > 0\}
Conditional probability
P(A \mid B) = \frac{P(A \cap B)}{P(B)}
Divisibility relation
3 \mid 12 \text{ means '3 divides 12'}