The word
Adagio when used in
music notation used to mean
that the music should be played in a
leisurely way or according to
the musician's own wishes.
Baroque Adagios even left some space for
improvisation. Around the
18th century however, it had come
to mean just '
slow'.
There are quite a few musical pieces named Adagio, but the most
well-known and the one most people mean is Adagio in G minor
by Tomaso Albinoni. It has been used and abused over
and over again, and you've probably heard it as muzak played in a
super market. The Doors used it as musical backdrop in the song
The Severed Garden.
The problem is, Albinoni (who lived in the 17th century) didn't
actually write the piece himself. It was in fact put together by the
20th century musicologist Remo Giazotto, who based it on material
attributed to Albioni.