WordNet sense 1 ((music) melodic subject of a musical composition):

idea, melodic theme, musical theme

WordNet sense 2 (the subject matter of a conversation or discussion):

subject, topic, subject

WordNet sense 3 (an essay (especially one written as an assignment)):

composition, paper, report, report

WordNet sense 4 ((linguistics) the form of a word after all affixes are removed):

base, radical, root, stem, root word

WordNet sense 5 (a unifying idea that is a recurrent element in a literary or artistic work):

motif