On the fly

Colloquial usage
In colloquial use, "on the fly" means something created when needed. The phrase is used to mean:
 * 1) something that was not planned ahead
 * 2) changes that are made during the execution of some activity: ex tempore, impromptu.

ArcGIS software usage
In ArcMap software, data is displayed on a map using a specified coordinate system. When additional data is added to the map does that not use the same coordinate system, ArcMap can project this data "on the fly." That is, ArcMap can automatically transform the data's projection system to be the same as that used for the map.

Sports usage
In ice hockey, the majority of line changes occur "on the fly", that is, while play is still ongoing. Players leap over the boards as quickly as possible as their line is substituted in to avoid any prolonged shifts and to ensure that fresh players remain on the ice as often as possible.

Computer usage
A computer can compute results on the fly, or retrieve a previously stored result.

It can mean to make a copy of a removable media (CD-ROM, DVD, etc) directly, without first saving the source on an intermediate medium (a harddisk); for example, copying a CD-ROM from a CD-ROM drive to a CD-Writer drive.

When used for encrypted data storage, on the fly the data stream is automatically encrypted as it is written and decrypted when read back again, transparently to software. The acronym OTFE is typically used.

On-the-fly programming is the technique of modifying a program without stopping it.

A similar concept, Hot swapping, refers to on-the-fly replacement of computer hardware.