three-line whip
noun [ C usually singular ] uk/ˌθriː.laɪn ˈwɪp/ us/ˌθriː.laɪn ˈwɪp/
（政党领袖要求该党议员按照党的要求投票的）紧急指令
In the UK a three-line whip is an instruction given to Members of Parliament by the leaders of their party telling them they must vote in the way that the party wants them to on a particular subject.