The term, "On The Fly," in PC terms, is when a computer computes results in real time.
On The Fly is usually referred to when making copies of removable media (DVDs, CD, Blu-rays) without saving anything to hard drives first.
An example would be copying a DVD that is in one drive, while writing the same copied media to a second disc in a second drive.
The same term is used when encrypting data storage, when the data is encrypted while it is being written. For that, the term OTFE (on the fly encryption) is used.
