In my understanding, the common understanding of the word "*topic*" is as defined in the [Wiktionary](https://en.wiktionary.org/wiki/topic):

> 
> *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.*
> 
> 
>