Definio makes finding word definitions very convenient. 

The app uses spaced repetition and active recall study techniques which are suitable for individual users. The program also provides the user with the option of creating customised e-notebooks to organise their definitions into multiple categories. It also offers the option to categorise words by creating a separate e-notebook for every subject.

You can create an e-notebook for yourself and for any subject you wish to learn about. You can add the definitions and the categories you want to create an e-notebook for.