Does this qualify as "self plagiarism" or something? Over the last few years, I have been writing several papers in the same direction as part of a research program. This means that the same exact setup is introduced at the beginning of each of my papers: i.e. the basic assumptions on the categories I am using, the basic terminology and notions, etc. As you can imagine, there are only so many ways of stating the same exact assumptions over and over again in each paper and now the first one and half pages of all my papers are beginning to look very much alike. I am a little worried: is this some kind of misconduct? Even though the content of my papers is different, is this "self plagiarism" or something when the first part of the first section looks almost identical to that in other papers? 
I should mention here that by "first part of the paper", I am not talking about the introduction. Obviously, each paper has different motivations and different results and hence different introductions. I am talking about the first page of the body of the paper, where you put in stuff like "Let C be a category satisfying ....yada yada yada...and we will denote this operad  by this and so on..."
 A: I find the question very relevant. And I think that there is a big difference between Mathematics and other subjects. In Mathematics, we want definitions to be exactly the same. We want to define a topological space in exactly the same way in all papers dealing with topological spaces. And let us be frank there are only so and so many ways to define topological spaces. Once I have found a very nice way to define and present it, in a notation that follows the general tradition, is very readable etc, why should I rewrite it for another paper?
I know that some people say that maybe at some point you do not need to explain topological spaces anymore, but this is out of question. The exponential time hypothesis(ETH) is used in 1000's of papers and still reviewers usually require a formal definition again and again.
The point here is that the majority of Mathematicians are not considering a folklore definition as a substantial contribution of the paper in any way. Nevertheless, it might be necessary to include it in all papers, for reasons of self containment. 
It also makes little sense to quote yourself on it, as it became 
"folklore" for some of the community working with ETH.
I think this is very different to texts in other sciences as history, philosophy etc. So it would be nice, if there could be some realistic consensus on the subject of self-plagiarism for the field of Mathematics.
best
Till
A: In terms of misconduct, I imagine it can be mollified with adequate references and noting from where you are borrowing.  If it is clear that it is just setup material and not repeating the same results, you are including material for the convenience of the reader and or to make the paper self contained.
At this writing, more and more use is being made of repositories like the ArXiv.  I think it quite reasonable (contrary to Mariano's sentiments above) to make a 'travel version' of your setup which is compact and condensed for inclusion in a paper, and repeated for several papers, all containing a reference to an expanded version of the setup as part of an ArXiv paper which includes more motivation and discussion for the new reader.  This way you can accommodate both first-time and more experienced readers of your material, almost all of whom have access to ArXiv.
Gerhard "Make Your Work Go Places" Paseman, 2017.02.16.
