Pressure, within a parliamentary political party, to conform to that party's official position when voting on bills or resolutions.
The degree of discipline exercised by parties is usually a function of a country's (or province's or other political unit's) political culture and doesn't normally have much to do with the effectiveness of the government. Elected representatives in the United States are subject to very, very little party discipline, often crossing party lines several times a day to vote with the other side and suffering no formal repercussions -- mainly because the party with the most seats doesn't face losing its privileged position completely just because some of its members vote against it now and then. Governments built on the Westminster model, such as Britain, Canada and Australia, often feature much more disciplined parties, in which members of parliament can be kicked out of their parties for not voting the party line; under the system in those countries, a governing party can be kicked out of office if it loses a vote on one of its own bills, having "lost the confidence of the House."
In those countries, however, there are aften raucous arguments within the individual parties while their positions are being thrashed out, so it's not as undemocratic as it sounds. It's unusual for party leaders to decide their parties' positions by fiat. When the arguing is over, though, the parties' whips -- elected members responsible for party discipline -- get to work.
In some parliamentary democracies, parties -- especially governing parties, to whom being able to deliver a majority vote matters the most -- there are degrees of discipline applied to individual votes, depending on the importance of an issue. This is measured in "whips" (the metaphor being that the number of party officers assigned to get the voting members onside is a measure of the vote's importance to the party leadership) or sometimes "lines." A "three-whip vote" is usually a so-called "confidence measure," where a governing party that loses has clearly lost its ability to govern -- something like a budget bill, or a declaration of war. A two-whip vote is significant but party members who have major problems of conscience with the party's position, or represent constiuencies violently opposed to the measure, can deviate. A one-whip vote allows even more latitude, and a "free vote" lets members vote however they please.