Tombstone (typography)

for the Latin phrase "quod erat demonstrandum".

[1][2] In Unicode, it is represented as character U+220E ∎ END OF PROOF.

Its graphic form varies, as it may be a hollow or filled rectangle or square.

In his memoir I Want to Be a Mathematician, he wrote the following:[1] The symbol is definitely not my invention — it appeared in popular magazines (not mathematical ones) before I adopted it, but, once again, I seem to have introduced it into mathematics.

It is most frequently called the 'tombstone', but at least one generous author referred to it as the 'halmos'.