1. A resource that acts as a proxy for some subject; the topic map system's representation of that subject. The relationship between a topic and its subject is defined to be one of reification. Reification of a subject allows topic characteristics to be assigned to the topic that reifies it.
2. A <topic> element.
According to the definition by TopicMaps.Org Specification in XML Topic Maps (XTM) 1.0 (http://www.topicmaps.org/xtm/index.html )