Transfer a file from one local device to another, such as from a computer to an e-reader.
The word is a play on upload/download, which refers to sending data to or from a remote device.
Some e-readers restrict the types of files that can be sideloaded.
For example, an e-reader might allow DRM-free files to be sideloaded, while DRM-activated files must be acquired through an associated on-line store and kept on the device.
Some devices make sideloading user-friendly, while other devices treat it as an activity for advanced users only.