What is the definition of "research Gap"? and Which part of research body should fill that Gap?

== When we say "we attempt to bridge the gap between two issues ", it means that we built our efforts to span the differences or spaces between issues, in order to solve any problem will be produced, or to minimize as much as possible the differences between things to be closer and no further distinctions any more.
== If you are working on original research, you will want to identify a need for your research somewhere close to the beginning of your paper.
Why? Because you have to show the reader that you are not duplicating existing research.
In other words: this paper is worth reading! This is best done by surveying the current research and then identifying a gap that you are going to fill.
== A research Gap is the missing element in the existing research literature, and you have to fill with your research approach to make your manuscript publishable.
In another words, it indicates a finding from a research in which a key question has not been answered.
== Your manuscript's introduction develops the Gap to show how the literature leads to your approach.
If you want to write such introduction, you can configure out the common sequence for it in an academic journal article in the following steps: -
1- Starts with establishing the field you work on it; by identifying the broad problem and how you can state its importance.
This can be the motivating issue of your research.
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2- Writes the previous researches summary (literature) and states what is significant in what has already been written.
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3- Creates a research space or Gap.
By writing three short paragraphs describe your purpose to fill in the existing research literature, and finding an opportunity for you to make a contribution to the research in the area is well approached.
So, in writing those paragraphs, you have to consider the following concepts: - Mixed previous empirical results (positive/negative; significant/insignificant) - Conflicts in theoretical approach - Conflict in empirical methods - Arbitrage between literatures - Complex ( limited previous results, conflicts, arbitrage) -
4- To finalize your introduction' letter, you have to introduce your research project; by developing two short paragraphs that emphasize your approach in resolving the mixed results, conflicts or arbitrage, and this can be clearly declared by establishing your research thesis or questions.
== Once you have decided what area you want to target and you have found appropriate sources to research you will need to interpret the results.
Evaluate the information you have gathered carefully and if necessary modify your search.
As with all research, formulating questions that need further investigation and being able to identify gaps in the literature must be approached with research.
== In this instance you are looking specifically for gaps in the research where you can make your mark by suggesting ways forward or theories of your own, or merely just point out that research is incomplete in any given area.
Once you have outlined the gaps in the literature point them out with their flaws making sure that you outline pertinent issues for future study.
== Although pointing out the gaps in the literature may show that you have looked thoroughly at the area of study your own interpretation and suggestion on how research may proceed or what could be done to fill those gaps will be infinitely better.
== In case of writing Reviews; which are currently the optimal method to summarize existing research, and also identify key gaps in current research that preclude answering policy questions in research outcomes.
Reviews should be composed of limited literature describing organizing principles or frameworks for determining research gaps.
I hope that my answer would declare and fill the Gap of the addressed question, and fulfill its purpose.
best regards, Lubna Al-Sharif

to my knowledege research gap is identfication of opportunities that are yet to be explored in a research study. take for example, a research has been carried out about Lean transformation; the gap here entails what has not been critically analysed or future research opportunities. The phrase 'research gap' can be linked to a systematic review or critical review or mapping review/scope in order to find te gap/ opportunity. 

صحيح فيه كثير من الفجوات البحثية لكن بالتراكم المعرفي يكون سد الفجوة اكثر إمكانية وأسرع فعالية

thank you so much all of you............I think its useful to me... Again i want to determine what is the futuristic scope in use of biodiesel in ic engines...on which parameters should be focus for research.

It is the difference between what is existed (knowledge = theories, assumptions, practices) and what is required/targeted (or what should be done). Research gap is critical to determine need and necessity of conducting a research.  Generally, a research gap can be extension or challenge of the existing variable, theories and assumptions ... etc.