Definition:QED

Definition
The initials of Quod Erat Demonstrandum, which is Latin 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 is usually used instead.

ProofWiki universally uses to signify the end of a proof.

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