Main entry: topic, subject, matter, issue
Definition: some situation or event that is thought about
Usage: he kept drifting off the topic; he had been thinking about the subject for several years; it is a matter for the police
Main entry: subject, topic, theme
Definition: the subject matter of a conversation or discussion
Usage: he didn't want to discuss that subject; it was a very sensitive topic; his letters were always on the theme of love