On the fly

In relation to computer technology, on the fly describes activities that develop or occur dynamically rather than as the result of something that is statically predefined. For example, the content of a page that is sent to you from a Web site can be developed (and varied) "on the fly" based on dynamic factors such as the time of day, what pages the user has looked at previously, and specific user input. The Web server calls an program to produce the "on-the-fly" page that is to be returned. There are several techniques for on-the-fly page development, including the server side include, the use of cookies (information previously stored about you that is located in a special file on your hard disk), and Microsoft's Active Server Pages.

In colloquial use, it means at the moment.