To define your subject as clearly as possible (and to write a good essay) you need a clear research question and a clear approach or angle. This will enable you to make a selection from all the literature available on a subject. Formulating a good research question requires a great deal of thought and time. Still, it is absolutely rewarding to take your time for this part of the research process since your research question is the foundation for your research. In addition, a clear research question will also save time, becauses it enables you to tackle your research in a focused manner.
Generally, academic research results in an academic text, i.e. an article or a paper. A good text has a clear angle; the perspective from which the topic is approached. This angle helps the author to select material and to write a coherent text.
For more information on formulating a research question and deciding on an approach, go to the Academic Communication Skills portal of the University of Groningen Language Centre.
This example shows how you can make a search term more specific:
The topic is very broad. Your search will get too many hits. So next you make the period and the location more specific:
Flint discoveries in the North of the Netherlands after 1800
The topic can be made even more specific: Flint discoveries in Borger between 1800 and 1900
Finally you make your topic even more specific: What kinds of flint have been found in and around Borger between 1800 and 1900 and what were they used for.