It’s an official record written by embassy staff to the Department of State in Washington D.C. of an interaction between U.S. diplomatic corps and host country counterparts. The information included within the cable is a summary and important details of a meeting between U.S. diplomats and their counterparts.
Every embassy writes hundreds and hundreds of cables each day. Here’s an example of one…
“So-and-so met with so and so today in the ministry of agriculture and discussed water policy.”
via Yahoo Answers
Political messages communicated internationally may also be referred to as “cables,” derived from the days when an international telegram was sent via a cable (such as the transatlantic cable).
What is a diplomatic cable? (another thought)
A diplomatic cable is a diplomatic message. The etymology goes back to the mid-nineteenth century when the first diplomatic messages were sent via telegraph. Telegraphs were connected by telegraph cables and the focus on telegraph as the technical device was very often on laying and managing cables (e.g. the epic attempt to lay trans-Atlantic Cable in 1860s). via E-Diplomacy