Theme

n.
  1. A subject or topic on which a person writes or speaks; a proposition for discussion or argument; a text.
  2. Discourse on a certain subject.
  3. A composition or essay required of a pupil.
  4. (Gram.) A noun or verb, not modified by inflections; also, that part of a noun or verb which remains unchanged (except by euphonic variations) in declension or conjugation; stem.
  5. That by means of which a thing is done; means; instrument.
  6. (Mus.) The leading subject of a composition or a movement.