What is a “no-contest” clause?

The case of Nathan v Leonard concerned a “no-contest” condition within a codicil which had been made after the will. The issue of contention was over policy concerns regarding the clause, which stated that benefits should be forfeited if the will was challenged.
General Election 2017: What the manifestos mean for the Wills & Probate market

Among the many issues highlighted in the recently published manifestos, social care and later life planning have been the subject of much debate.