1. ------A melodic passage or phrase, especially in Wagnerian opera, associated with a specific character, situation, or element. A frequently repeated phrase, image, symbol, or situation in a literary work, the recurrence of which usually indicates or supports a theme. The term (German, ‘leading motif’) comes from music criticism, where it was first used to describe the repeated musical themes or phrases that Wagner linked with particular characters and ideas in his operatic works. The repeated references to rings and arches in D. H. Lawrence's novel The Rainbow (1915) are examples of the use of a leitmotif
2. ------Related to hexa (six), not to the word hex (curse), hexakosioihexekontahexaphobia, the fear of 666, makes for quite a chilling leitmotif, or theme — perfect for a scary cinematic experience:
"Hexakosioihexekontahexaphobia is a fear found in the Western Christian world, which originates in the belief that the Biblical verse, Revelation 13:18, indicates that the number 666 is linked to Satan or the Anti-Christ. Outside the Christian faith, the phobia has been further popularized as a leitmotif in various horror films."
3. -- ---Recurring themes or subjects in other forms of art or literature are sometimes also called leitmotifs.
Sources: YourDictionary; Answers.com