Leitmotif
Wagner did not actually use the word “leitmotif”.  He preferred to call such themes “Grundthema” (basic idea) or simple “Motiv”.  Some people, such as Eduard Hanslick, who did not like Wagner’s music, thought that Wagner’s use of leitmotifs made the music too simple.  Claude Debussy said the use of leitmotifs in operas was like having a world where crazy people would always use "visiting cards" to introduce themselves and then start singing their names to be sure everyone knew what was going on.