a definition of topic

Here's my first offering of a 
definition:

  
Topic
A body of 
  content that develops a single subject in a self-contained way. A series of 
  topics may develop a complex subject, but it develops one aspect of that 
  subject at a time, each of which is a topic. A topic is formally characterized 
  by limited vocabulary pertaining to its subject and by a unity of voice and 
  style. Typically, the subject is identified in a title and briefly delineated 
  in a first paragraph, although these characteristics may not be universal to 
  all topic types.

Here is a definition from the spec (2.1.1.3 Basic 
concepts, 2.1.2.1.1 The topic as the basic unit of 
information):

  
In DITA, a topic is 
  the basic unit of authoring and reuse. All DITA topics have the same basic 
  structure: a title and, optionally, a body of content. Topics can be generic 
  or more specialized; specialized topics represent more specific information 
  types or semantic roles, for example, <concept>, <task>, 
  <reference>, or <learningContent>. 

  
 

  
[...]

And farther along in 2.1.2.1.1:

  

  
Classically, a DITA topic is a titled unit of information that can be 
  understood in isolation and used in multiple contexts. It should be short 
  enough to address a single subject or answer a single question but long enough 
  to make sense on its own and be authored as a self-contained unit. However, 
  DITA topics also can be less self-contained units of information, such as 
  topics that contain only titles and short descriptions and serve primarily to 
  organize subtopics or links or topics that are designed to be nested for the 
  purposes of information management, authoring convenience, or 
  interchange.