When you do something with assiduity, you really focus your attention on it. If you work with assiduity on a research paper, you're sure to get an A.
If you've ever watched professional tennis, you've seen players focus with assiduity on their game. Diligent students, serious chess players, and successful businessmen all make great use of their assiduity to get what they want and do their best work. The noun assiduity has been around since the fifteenth century, and it comes from the Latin assiduatem, or "continual presence."