When a mathematicaian writes QED at the end of a proof what does it mean?

When a mathematician writes "QED" at the end of a proof, it stands for the Latin phrase "quod erat demonstrandum," which translates to "which was to be demonstrated." This notation signifies that the proof is complete and that the statement has been successfully established. Traditionally, it serves as a formal conclusion, indicating that the arguments presented have logically led to the desired result.