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.
Sideloading typically refers to media file transfer to a mobile device via USB, Bluetooth, WiFi or by writing to a memory card for insertion into the mobile device.
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.