Leitmotif
The use of leitmotif was not completely new in the 19th century.  For example: the first four notes of Beethoven's Fifth Symphony are used like a leitmotif during the whole symphony.  It was, however, Carl Maria von Weber who first used it a lot in his operas.  A music critic called F. W. Jähns used the word to describe Weber’s work.  Hector Berlioz wrote a symphony called Symphonie Fantastique which has a tune which he called idée fixe (“fixed idea”) to represent the love between the two characters.