A procedure used in both houses of Congress to bring to the floor the number of Members of the House or Senate who must be present for it to conduct its business.
In the Senate, a Senator who has the floor can force a quorum call at almost any time by suggesting the absence of quorum. The presiding officer usually cannot count to determine whether or not a quorum is present. So when a Senator “suggests the absence of a quorum,” the presiding officer directs the Clerk to call the roll of Senators aloud by name. If a majority of Senators respond, a quorum is present and the Senate can return to its business.
However, a quorum call in the Senate usually has a different purpose: most often it is a strategic move that is used to delay proceedings for a variety of reasons — for example, to conduct informal negotiations on or off the Senate floor, or to await a Senator who is expected to make a speech or propose an amendment. If the purpose of a quorum call actually is to bring a majority of Senators to the floor, it is known as a “live” quorum call.
A quorum call in the House seeks to bring a majority of Members to the floor to record their presence after the absence of a quorum has been established. In the House, a Member makes a point of order that a quorum is not present, usually only when a vote is taking place. The Speaker (or the Chairman of the Committee of the Whole) then counts to determine if a quorum is present. If a majority of Members fail to respond to a quorum call, the House must adjourn or take steps to secure the attendance of enough Members to constitute a quorum.
Source: Congressional Glossary