The word comes from music and it is characterized by the recurrence of a short musical phrase. Of German origin: from leit- "leading"+motiv "motive". Adopted in literature, it also refers to a recurring piece.
To identify a leitmotif one needs, indeed, to look for recurrent motifs. However, the repetition is not the only criterion. A leitmotif is a piece that holds great significance. It usually emphasizes an idea, or offers a different perspective upon the literary work. A leitmotif can be an image, an event, an object, a place, a character, but in essence, it is an idea. For example, in the epic poem “The Song of the Nibelungs” the leitmotif is the idea that after happiness comes pain, common for the medieval thinking. On the other hand, in "Gone with the Wind" by Margaret Mitchell, the leitmotif is Tara, or the land. Although it is a more perceptible image, it also stands for an idea. The recurrence of Tara emphasizes the importance of the land and its connection with Scarlett. It also links Scarlett with her Irish ancestors.
"The land is the only thing in the world worth working for, worth fighting for, worth dying for, because it is the only thing that lasts" Gerald O'Hara in "Gone with the Wind"
Other examples of leitmotifs would be :
War and its misfortunes, in "War and peace", by Leo Tolstoy. This is a leitmotif not only because war influences the action of the novel, but because the author introduces different perspectives on war and its misfortunes through his characters.The searching of the self in “In Search of Lost Time” by Marcel Proust. The eye of God in „The Legend of the Ages”, a collection of poems by Victor Hugo. Penitence and redemption in „Robinson Crusoe” by Daniel Defoe.
A leitmotif is the whispering voice of the author instilling ideas and broadening perspective.