\begin{definition}[Definition:QED]
The initials of '''Quod Erat Demonstrandum''', which is {{WP|Latin|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 $\blacksquare$ is usually used instead.
{{ProofWiki}} 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.
\end{definition}