theme

– noun
1. a subject of discourse, discussion, meditation, or composition; topic.
2. a unifying or dominant idea, motif, etc., as in a work of art.
3. a short, informal essay, esp. a school composition.
4. a. a principal melodic subject in a musical composition.
b. a short melodic subject from which variations are developed.
5. the element common to all or most of the forms of an inflectional paradigm, often consisting of a root with certain formative elements or modifications.