Fundamental research is a research that is conducted to test a theory that is fundamental of a scientific discipline.
Fundamental research is often seen as very theoretical and the opposite of applied research. However, the distinction is less strict: applied research can also be very scientific and fundamental. It is often said that the distinction has to do with the direct applicability of the results of research. Two forms can traditionally be distinguished: research that must yield directly usable knowledge (because decisions must be made for which this knowledge is important) and research that is carried out purely and solely to increase knowledge. The first is referred to as problem-solving or practice-oriented research and the second is always referred to as fundamental research.
It is also often said that fundamental research is mainly conducted to satisfy curiosity. It is not of immediate use, which does not mean that it can never be useful. It could prove to be of great value in the future. That future does not have to be far away. Fundamental research into the composition of atoms can be important in the short term for energy supply, but perhaps never.
In my opinion, this vision is incorrect. Whether something makes sense in practice and whether it matches a theory are not opposites, but two independent dimensions. Every research has a theoretical and a practical component. In my opinion, the best research is research that scores high on both dimensions.