Definition:Halmos Symbol

Definition
The Halmos symbol is the character: $\blacksquare$ used to indicate the end of a proof.

It replaces the old-fashioned and embarrassingly uncool Q.E.D. which muggles sometimes use when pretending to be clever.

Also defined as
Some sources show this, in some contexts, as a heavy vertical line, something like: $\Rule{3px} {2ex} {0ex}$ or $\Rule{5px} {2ex} {0ex}$, rather than a box, but the intention is the same.

This form is used, for example, by in his  to indicate the point of termination of an algorithm.

Some sources use $\Box$ in preference to $\blacksquare$.

However, prefers to reserve the former symbol for the end of a subproof within a long proof, as a way of breaking up the flow of thought for clarity.