The Carlisle principle is a term coined by Prime Minister David Cameron on 20 April 2015 in the run-up to the May 2015 United Kingdom general election, 2015 as a tag for the principle that checks and balances are required to ensure that devolution to Scotland has no adverse effects on other parts of the United Kingdom.
The phrase references a fear that Carlisle, being the English town closest to the Scottish border, could be affected economically by preferential tax rates in Scotland.
References
Carlisle principle Wikipedia(Text) CC BY-SA