Theme, subject (5)
I believe the answer is:
topic
I believe this is a double definition.
'theme' is the first definition.
(I know that theme can be written as topic)
'subject' is the second definition.
(as in a topic for discussion)
(Other definitions for topic that I've seen before include "Subject (of speech, etc)" , "Issue (for discussion)" , "Subject (for conversation, etc)" , "Subject (of, eg, speech)" , "That may be debatable" .)