Notion is a notetaking software and project management software that is used for note-taking, task management, project management, knowledge management, and personal knowledge management. The app uses databases and markdown pages for use in personal and collaboration work.
This is a bit unwieldy, but so is Notion, which does have a bit of a learning curve. Wikipedia also calls notion productivity software which is also a bit generic though also correct. That said, the high degree of functionality and clean aesthetic is essentially a next generation wiki+docs+data. It is an evolutionary leap from the Google Drive Suite (a successor to the Microsoft Office Suite), as it is all a single tool rather than different editors, and the documents live inside the app. This is very much a web 2.0 approach, which works very well.
For what can Notion be used?
A definition is not as useful, but rather it is a sense of what can one do with Notion, that is important. Essentially it is an information management tool but that also doesn't say enough. It is how one can organize one's own information -- personal information management (PIM) -- and it includes sharing and collaboration, so it is quite useful for teams.