A filled square symbol commonly used to mark the end of proofs or as a solid box operator in logic.
Examples
End of proof marker (QED symbol)
\text{Therefore, } x^2 + y^2 = z^2 \text{ holds for all Pythagorean triples.} \blacksquare
Logic truth table (filled square for true)
p \land q = \blacksquare \text{ when } p = \blacksquare \text{ and } q = \blacksquare
Set theory diagram element
A = \{\blacksquare, \square\} \text{ where } \blacksquare \text{ represents a filled element}