From Longman Dictionary of Contemporary Englishthree-line whipˌthree-line ˈwhip noun [countable] British English 1 PPVan order from a leader of a British political party telling MPs in that party that they must vote in a particular way2 informal a very firm instruction that people must come to a meeting or take part in an event
Examples from the Corpus
three-line whip• Labour should impose a three-line whip to vote against the Treaty.• On Thursday, St Valentine's night, Tommy was on a three-line whip.• Many Opposition Members who will obey the three-line Whip and vote against the motion will do so with a heavy heart. 