Data Set Definition , Where to Place (Introduction, Material and Methods)?
There is no universally-agreed way to do it. I usually place the information about the datasets under the experiments section, before reporting the results. Also, I prefer to present parameters about the datasets (e.g., number of features, number of samples) in a table. If many datasets are used, and there are some points that needs to be clarified, I would create a subsection for the datasets under the experiments section.