pp. Transferring music or other content to a cell phone using the cell phone provider‘s network.

Example Citations:
Consumers in Japan, South Korea and Great Britain are already transferring songs directly from their computers to their phones, a practice known as sideloading, or downloading full-length tracks over their mobile networks.

Ringtones may be big in the United States now, but full-track downloads and sideloading are the future, said Thomas Hesse, president of global digital business at Sony-BMG Entertainment.
—Alex Veiga, "Wireless carriers banking on mobile music," The Associated Press, September 23, 2005

Carriers initially prohibited the practice of allowing users to transfer existing music from their personal digital libraries to MP3-capable phones — a process known as "sideloading" — but they are now warming up to the idea.

Motorola's iTunes ROKR phone is the latest and best-publicized example. Cingular is not only supporting the sideloading capabilities, it is encouraging it with a rash of TV, billboard and print ads mimicking Apple's successful iTunes/iPod campaign.
—Antony Bruno, "The Next Move for Mobile Music," Billboard, October 1, 2005

Earliest Citation:
With the industry now adjusted to the concept of paid-for downloads, high street chains are steeling themselves against new claims on their market share. That may mean consumers "sideloading" from their digital provider to their mobile phone (witness Napster's new partnership with Ericsson, and iTunes' association with Motorola).
—Paul Sexton, "Storm in a coffee shop," Financial Times (London), June 21, 2005

Sideloading is a variation on the more familiar terms downloading — receiving data from a remote source — and uploading — sending data to a remote source. The medium for most downloading and uploading activity these days is the Internet, so this sense of sideloading is used to refer to downloads that occur over the cell phone provider's network (off to the side, as it were).

Sideloading can also mean sending downloaded data to a memory card or other removable storage medium:

Verizon customers can use their PCs to download music to standard memory cards, a process called sideloading, and insert the cards into their phones to listen to music, Verizon said.
—"Verizon Lets Users Download MP3s to Phone Memory Cards," Warren's Washington Internet Daily, September 7, 2005
Yet another sense of the word refers to transferring data from one server to another:

An exclusive feature of the myplay service, myplay’s "Copy To Locker" technology allows the user to "sideload" tracks from partner sites directly into their Locker. Unlike uploading or downloading, "Copy To Locker" transfers the music file instantly from server to server, eliminating lengthy and complicated downloading processes. This makes it easy for the consumer to rapidly build an online music collection from a variety of sources.
—"New myplay free tracks include Fastball's "You're an Ocean," Corporate News Net, October 23, 2000

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