Mozart's Requiem. Verdi's Requiem. The German Requiem by Brahms. War Requiem Britten. Everyone probably heard these words, but did not think of what they mean.
In Latin requiem aeternam - eternal peace. These words are the beginning of Catholic prayers for the dead. Once composers wrote this Latin canonical, that is legalized, never-changing the text of the hymns for soloists, choir and organ or orchestra. Performed these songs in the temples, during the funeral service.
C. A. Mozart. The Requiem. Part 8. Lacrimosa (Tear)
From the time of Mozart's Requiem loses cult - Church character, but wrote it on the Latin text. It is used even in Verdi's Requiem, which in music is more like Opera than a Church service. "A German Requiem" by Brahms is so-called because the composer first refused the Latin text, replacing it by the words in their native German language.
In our time the concept of the word "Requiem" has greatly expanded. Great English composer B. Britten called his monumental anti-war work in which, in addition to Latin, sound passionate poems of the English poet Wilfrid Owen, who was killed during the first world war.
After the poet Robert Rozhdestvensky Requiem called his vocal-symphonic work written on his verses, D. B. Kabalevsky. Of course, there are no Latin texts, prayer, and anything associated with religion. Requiem Kabalevsky dedicated to those who died in the struggle against fascism in the great Patriotic war. He curses the war, calls for the struggle for peace, for the bright future of children.
Often called Requiem separate small vocal-symphonic or orchestral works or episodes within the larger works - the Opera or the Symphony, when they dedicated to the memory of the fallen heroes and are sorrowful sublime, focused nature.