From the XTM specification:
A topic is a resource that acts as a proxy from some
subject; it is the topic map system's representation of that
subject.
A subject is anything that can be spoken about or conceived of by a human being. In the most generic sense, a subject is anything whatsoever, regardless of whether it exists or has any other specific characteristics, about which anything whatsoever may be asserted by any means whatsoever. In particular, it is anything about which the author of a topic map chooses to discourse.