Which scientific discipline studies topics? In my understanding, the common understanding of the word "topic" is as defined in the Wiktionary:


  Subject; theme; a category or general area of interest.


Now which discipline deals with this and related effects, for example how to determine topicality of a message in a given context? 

First, I thought it could be linguistics, but this discipline has got their own notion of topic, either they work on a single-sentence level:


  In linguistics, the topic, or theme, of a sentence is what is being
  talked about.
Sociology could be one consideration. The word “topic” has the ability to shift, what could be considered a “theme” is dynamic, and evolves with societies definition of what it means. Philosophy could also be a major player because you’re concerned with the question of what it means to be a topic, and that sounds like the realm of philosophical thought. To me it sounds more like a question best suited for the social sciences as opposed to the hard sciences.