Sideloading is a term used mostly on the Internet, similar to "upload" and "download", but in reference to the process of transferring files between two local devices, in particular between a computer and a mobile device such as a mobile phone, smartphone, PDA, tablet, portable media player or e-reader.
When referring to Android apps, "sideloading" typically means installing an application package in APK format onto an Android device. Such packages are usually downloaded from websites other than Google play, usually through a computer. Side loading of apps is only possible if the user has allowed "Unknown Sources" in their Security Settings.
When referring to iOS apps, "sideloading" means installing an app in IPA format onto an Apple Device, usually through the use of a computer program such as Cydia Impactor  or Xcode or on the actual device using a Jailbreak method or using a signing service instead of through Apple's App Store. On modern versions of iOS, the sources of the apps must be trusted by both Apple and the user in "profiles and device management" in settings; except when using jailbreak methods of sideloading apps.
The term "sideload" was coined in the late 1990s by online storage service idrive.com as an alternative means of transferring and storing computer files virtually instead of physically. In 2000 idrive applied for a trademark on the term. Rather than initiating a traditional file "download" from a website or FTP site to their computer, a user could perform a "sideload" and have the file transferred directly into their personal storage area on the service. Usage of this feature began to decline as newer hard drives became cheaper and the space on them grew each year into the gigabytes and the trademark application was abandoned.
The advent of portable MP3 players in the late 1990s brought sideloading to the masses, even if the term was not widely adopted. Users would download content to their PCs and sideload it to their players.
Today, sideloading is widespread and virtually every mobile device is capable of sideloading in one or more ways.
Sideloading has several advantages when compared with other ways of delivering content to mobile devices:
Sideloading also has disadvantages:
Sideloading over a USB connection was standardized by OMTP in late 2007. Until this time, mobile phone manufacturers had tended to adopt proprietary USB transfer solutions requiring the use of bundled or third party cables and software.
Transfer performance of USB sideloading varies greatly, depending on the USB version supported, and further still by the actual engineering implementation of the USB controller. USB is available in Low-Speed, Full-Speed, and Hi-Speed levels, with High-Speed USB transferring up to 480 Mbit/s (60 MB/s). However, the majority of mobile phones as of the time of writing of this article are Full-Speed USB. Of the mobile products supporting USB 2.0 Hi-Speed, the actual sideloading performance usually ranges between 1-5 MB/s. However, the popular BlackBerry mobile phones by RIM and the iPods by Apple distance themselves at higher performing speeds of roughly 15.7 MB/s and 9.6 MB/s, respectively.
Bluetooth’s OBEX/OPP profiles allow for file transfer between a PC and a mobile device. Using this option is slightly more complicated than using a USB connection as the two devices have to be paired first. Also, unlike the familiar drag and drop that is usually available via USB, Bluetooth implementation is specific to the Bluetooth transceiver and drivers being used. Files that are sideloaded to mobile devices via Bluetooth are often received as messages, in the same way that SMS texts would be received. While these files can be saved to any storage medium, their initial location is the handset’s internal memory. As such the limitations of the internal memory have to be taken into account before beginning the sideload.
Sideloading via a memory card requires that the user have access to a memory card writer. Audio and video files can be written directly to the memory card and then inserted into the mobile device. This is potentially the quickest way of sideloading several files at once, as long as the user knows where to put the media files.