A collection of "topics", their relationships, and information sources. A topic map captures the subjects of which information sources speak, and the relationships between them, in a way that is implementation independent.
A topic is a symbol within the computer that represents something in the world such as the play Hamlet, the playwright William Shakespeare, or the "authorship" relationship.
Topics can have names. They can also have occurrences, that is, information resources that are considered to be relevant in some way to their subject. Topics can play roles in relationships.
Thus, topics have three kinds of characteristics: names, sources, and roles played in relationships. The assignment of such characteristics is considered to be valid within a certain scope, or context.
Topic maps can be merged. Merging can take place at the discretion of the user or application (at runtime), or may be indicated by the topic map's author at the time of its creation.