What Exactly is a 'Diplomatic Cable'?

This is the 21st century, so why are we still talking about State Department cables?

The Slate Magazine Explainer column explains just that in the 29 November 2010 article "What's a Diplomatic Cable? And why is the State Department still using them?" With help from the National Archives and State, author Brian Palmer discovers that there really isn't much functional difference between email and State's diplomatic cables. The word "cable" is used nostalgically to refer to the internal messaging system that has replaced the old undersea cable network. 

[h/t @dcpest]