*Quod Erat Demonstrandum*, a Latin translation of the phrase $\ddot{o} \pi \varepsilon \rho \quad \ddot{\varepsilon} \delta \varepsilon \iota \quad \delta \varepsilon \hat{\iota} \xi \alpha \iota$ with which Euclid concluded most of his proofs. It means "what was to be proved" and signifies the proof has been concluded.