Definition:QED

Definition
The initials of Quod Erat Demonstrandum, which is for which was to be demonstrated.

These initials were traditionally added to the end of a proof, after the last line which is supposed to contain the statement that was to be proved.

The usage is considered hopelessly old-fashioned nowadays, and is rarely seen outside nerd sitcoms.

The Halmos symbol $\blacksquare$ is usually used instead.

universally uses $\blacksquare$ to signify the end of a proof.

The symbol $\Box$ is used to signify the end of a stage part way through a proof, where a subsidiary result is proved.

Also known as
Some sources render it as Q.E.D. (that is, with dots).