The phrase "default wallpaper" refers to the pre-set background image or design that appears on a computer or mobile device when no other image or customization is chosen by the user. Full definition
Images such as default wallpapers showed noticeable color banding as well. (cnet.com)
The only real customization you'll notice in the UI is the Echo's elegant default wallpaper, which features a view of the Earth from outer space. (laptopmag.com)
This update brings a new default wallpaper that sports Material Design, automatic syncing of wallpapers between devices, and a new «Emoji palette» that will allow users to quickly access Emoji. (9to5google.com)