A relational symbol representing a frowning face or negative relation, commonly used to denote non-equivalence or dissatisfaction in logic.
Examples
Expressing 'not similar to' relation
A \frown B \text{ means A is not similar to B}
Negative relation in topology
x \frown y \text{ indicates x is not connected to y}
Expressing dissatisfaction in mathematical logic
P(x) \frown Q(x) \text{ denotes P is incompatible with Q}