Is sideloading same as flashing via custom recovery? Recently, I side-loaded the latest nougat release on my nexus 6p. Which was almost identical to how one would go about flashing via recovery. This brings me to the question- is there any off  the hand differences between side-loading a zip file against flashing it?
ROM Flashing refers to flashing the OS ROM. Usually involves installing a new image of the OS.

Side-loading usually refers to installing a single app that is not delivered from the Google Play Store (or whatever the app store your device is setup to use.) It can still flash ROMS, but it requires the process of using adb. Good for devices with limited storage, since the ROM can be on the PC with adb rather than the device, but sorta identical to Flashing otherwise.