Flatpak is a framework for building, distributing and installing apps. It allows the creation of apps that can be installed on multiple distros, and that are secure. For more information about Flatpak, see its website and wiki.
The Flatpak initiative is cross-distro and cross-desktop, but it has its origins in the GNOME project and is strongly supported.
Flatpak and GNOME
There are various efforts to utilise Flatpak within the GNOME project. This includes: