String inconsistency: repository/source/channel

[Sebastian Heinlein]

There is no clear use of the terms "repository", "source" and "channel".

Could anyone please clarify which term to use. A prompt decision would
be important for documentation and translation.

Regards,

Sebastian

Related apps:
 gnome-app-install
 update-manager 
 gnome-software-proerties
 synaptic
 and corresponding documentation

Related specs:
 https://wiki.ubuntu.com/ThirdPartyPackages
 https://wiki.ubuntu.com/RepositoryDialogRedesign