What is Party Discipline?

Legal Definition
Party discipline is the ability of a parliamentary group of a political party to get its members to support the policies of their party leadership. In liberal democracies, it usually refers to the control that party leaders have over their caucus members in the legislature. Party discipline is important for all systems of government that allow parties to hold political power because it determines the degree to which the governmental infrastructure will be affected by legitimate political processes.

The term has a somewhat different meaning in Marxism–Leninist political systems such as the People's Republic of China. In this case it refers to administrative sanctions such as fines or expulsion that the Communist Party can impose on its members for actions such as corruption or disagreeing with the party.

Breaking party discipline in parliamentary votes can result in a number of penalties for the member who dissents, These penalties include not being promoted to a cabinet position, and losing other perks of elected office like travel. Their disagreement with their party caucus may be so strong that they leave the party to join another parliamentary caucus or become an independent. This is known as crossing the floor.

In many political systems a member of each party is officially designated or elected as a "whip," whose role it is to enforce party discipline.
-- Wikipedia