It refers to a system of rules of conduct that customarily governs behavour of all members belonging to a party and are expected to abide by it.

the control used by a political party to encourage its members in Parliament to vote together

The convention that all MPs within any party vote together, as predetermined in the party caucus and enforced by the party whip.

  Modern parties are disciplined in the sense that, once elected to a representative assembly under the label of a party, it is expected that the member will adhere to the general principles and platform of the party and will vote with his or her colleagues in the parliament.  If the member does not conform to the decisions of the party, he or sh [..]

The control exercised by a party over the positions held by its members and over the way in which they vote. The enforcement of party discipline is one of the whip's duties.

Party discipline means upholding the entrenched principles, rules and regulations guiding the behavior of members of political party

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 con [..]