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 known as
The Halmos symbol is apparently, according to himself, also known as the tombstone symbol.

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.