THE BACKSTOP!

The BBC Radio 4 definition on 12.3.19: "The Backstop is the mechanism designed to fulfil the pledge made by the UK and the EU that the Irish border will remain open after Britain leaves.  It will kick in if a trade agreement with Brussels was not reached at the end of the post Brexit transition period.  It is contentious because it involves, in part, Northern Ireland and the rest of the UK temporarily remaining in a customs union with the EU."

Journalists, like Jeremy Vine, have described it as ensuring a "no wall between the ROI and NI.  ... there must be no physical border.  It leaves NI closer to the EU and the ROI than the UK."