the central idea that is expanded in a document or discourse(noun.cognition)
source: wordnet30
That which is borne or carried; a load.(noun)
source: webster1913
The verse repeated in a song, or the return of the theme at the end of each stanza; the chorus; refrain. Hence: That which is often repeated or which is dwelt upon; the main topic; as, the burden of a prayer.(noun)