WordNet (r) 3.0 (2006):

subject matter
    n 1: what a communication that is about something is about [syn:
         message, content, subject matter, substance]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "subject matter":
   argument, basis, burden, case, chapter, concern, essence,
   focus of attention, focus of interest, gist, head, heading, issue,
   living issue, main point, matter, matter in hand, meat, motif,
   motive, point, point at issue, point in question, problem,
   question, rubric, subject, subject of thought, substance, text,
   theme, topic