[alejoar]

I wonder where the term started?

Android itself calls it "install" when you open an APK file, there's not mention of "sideload" in Android at all as far as I can tell.

[viernullvier]

There is, actually, but in a different context. The `adb sideload` command allows you to boot a device from an image without flashing it.

[chasil]

This command is also used to install 3rd-party ROMs.

There is an option in the TWRP recovery tool to sideload any capable .ZIP file.