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 same activity: ex tempore, impromptu.

Automotive usage

In the automotive industry, the term refers to the circumstance of performing certain operations while a vehicle is driven by the engine and moving. In reference to four-wheel drive

Four-wheel drive

Four-wheel drive, 4WD, or 4×4 is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously...

 vehicles, this term describes the ability to change from two to four-wheel drive while the car is in gear and moving. In some convertible

Convertible

A convertible is a type of automobile in which the roof can retract and fold away having windows which wind-down inside the doors, converting it from an enclosed to an open-air vehicle...

 models, the roof can be folded electrically on the fly, whereas in other cases the car must be stopped.
In harvesting machines, newer monitoring systems let the driver track the quality of the grain, while enabling them to adjust the rotor speed on the fly as harvesting progresses.

Sports usage

In sports, "on the fly" refers to actions performed while play is still ongoing. For example, in ice hockey

Ice hockey

Ice hockey, often referred to as hockey, is a team sport played on ice, in which skaters use wooden or composite sticks to shoot a hard rubber puck into their opponent's net. The game is played between two teams of six players each. Five members of each team skate up and down the ice trying to take...

, the majority of line changes occur on the fly, or while the game is still in session; 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. Such changes can also be made in lacrosse.

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

CD-ROM

A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....

, DVD

DVD

A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

, 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

Optical disc recorder

In computing, an optical disc drive is a disk drive that uses laser light or electromagnetic waves near the light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only read from discs, but recent drives are commonly both readers and recorders,...

 drive. The copy process requires each block of data to be retrieved and immediately written to the destination, so that there is room in the working memory to retrieve the next block of data.

When used for encrypted data storage, on the fly the data stream

Data stream

In telecommunications and computing, a data stream is a sequence of digitally encoded coherent signals used to transmit or receive information that is in the process of being transmitted....

 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

Hot swapping

Hot swapping and hot plugging are terms used to describe the functions of replacing computer system components without shutting down the system...

, refers to on-the-fly replacement of computer hardware.