DDS Foundation Wiki

A Topic is the most basic description of data that is to be published and/or subscribed to. A topic connects a data writer with a data reader, i.e., communication does not occur unless the topic published by a data writer matches a topic subscribed to by a data reader. Communication via topics is anonymous and transparent, i.e., publishers and subscribers need not be concerned with how topics are created or who is writing/reading them since the DDS DCPS middleware manages these issues. In programming terms, a Topic is a Class and each instance is a Sample. Topic instances are associated with a key (similar to a packet ID), defined by IDL and a set of Quality of Service parameters.

Source: OpenSplice Glossary