Preliminari is perfectly fine (and if it brings to mind only sexual references you should probably read less magazines... :)). Furthermore it is already widely used.
Other alternatives are
- Prerequisiti: This has a somewhat scholastic tone. I'm not sure I'd use it in a scientific paper but on the other hand maybe yes.
- Fondazioni: I'm a bit uncertain about this but I wanted an alternative that emphasizes the "foundational" aspect of the English word background. Moreover I could swear I've seen it used like this somewhere, although I do not remember the exact place.
And remember: a scientific paper should be also enjoyable to read and not overly formal. There are already too many boring papers.
EDIT: I'll add a couple of sentences about how I see the Background section of the paper, so that it might be clear the context of this answer. In my mind it is different from the Introduction, where you briefly explain the contents of the paper without many details (of course this might involve the exposition of some background). Instead it is the section where you quickly review material which, while present in the literature, cannot be taken for granted but it is nonetheless important for the remainder of the paper. Let me give an example of the structure of an imaginary paper:
1) Introduction: This paper is about descending thick subcategories along flat maps. This is important because blah blah and it will be done by realizing the simplicial objects as blah blah...
2) Background on GHM obstruction theory: Let us quickly review the foundations of GHM obstruction theory. A more comprehensive account can be found in . Blah blah...
3) The main theorem ...