After I read about 40 papers in my research topic, I summarized 25 of them in my thesis, explaining methods used and solutions provided in the literature survey and literature-review sections. I found out a gap in the previous researches and would like to point it out in my thesis, in order to explain where my work would fit.
Should I call this section research gap, or there are a better scientific terms usually used?
"Literature review" would be a good term. It refers to the section in a paper where you review the existing literature and show where your new piece of research fits in.
Don't confuse this with a "review article", which summarizes and integrates a large number of previous publications, typically far more publications than you'd review for an original article. A review article typically only does the synthesis, and does not add any original research of its own, although it may well point out gaps in existing research for subsequent investigations.
In you want to draw specific attention to the fact that the question you will investigate has not been covered in previous research, you could call it "open questions".
The standard phrase is "a gap in the literature."
Open problem, unsolved problem, need for research, unexploted potentials, areas not covered by previuos research, questions not asked in previous reseach,
I found out a gap in the previous researches and would like to point it out in my thesis, in order to explain where my work would fit.
Should I call this section research gap, or there are a better scientific terms usually used?
There are two parts to this.
First is the literature review, or summary of existing literature. These are short summaries of the 40 papers you have researched and descriptions of their key points.
Second, you might want to have a section called research questions or research issues. This can take the form of statements like, "existing literature does not appropriately address X, Y, or Z" or "existing literature presents weaknesses for addressing A, B, C" types of things. Basically building the case for why your thesis exists. What problems with existing literature are you trying to solve?
"Knowledge gap" is another way to express that there is something missing from the literature.
Contribution
This would also work as a (sub-) section heading. I prefer it to "research gap/lacuna/desideratum" etc., because it focuses on what your research does to close the "gap".
The meaning of differentiation in [dictionary.cambridge.org][1]is given as :
"the act of showing or finding the difference between things that are compared".
So I suggest you may use the phrase "The research differentiation". [1]: https://dictionary.cambridge.org/
