"Q.E.D." (sometimes written "QED") is an abbreviation for the Latin phrase "quod erat demonstrandum" ("that which was to be demonstrated"),
a notation which is often placed at the end of a mathematical
proof to indicate its completion. Several symbols are occasionally
used as synonyms for Q.E.D. These include a filled square 
(Unicode U+220E, as used in Mathematics Magazine
and American Mathematical Monthly), a filled rectangle (Knuth 1997, pp. 3
and 39), or an empty square 
.
Q.E.D.
See also
Proof, Q.E.F.Explore with Wolfram|Alpha
References
Knuth, D. E. The Art of Computer Programming, Vol. 1: Fundamental Algorithms, 3rd ed. Reading, MA: Addison-Wesley, pp. 3 and 39, 1997.Referenced on Wolfram|Alpha
Q.E.D.Cite this as:
Weisstein, Eric W. "Q.E.D.." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/QED.html