The Senate tradition of unlimited debate has allowed for the use of the filibuster, a loosely defined term for action designed to prolong debate and delay or prevent a vote on a bill, resolution, amendment, or other debatable question. Prior to 1917 the Senate rules did not provide for a way to end debate and force a vote on a measure. That year, the Senate adopted a rule to allow a two-thirds majority to end a filibuster, a procedure known as "cloture." In 1975, the Senate reduced the number of votes required for cloture from two-thirds of senators voting to three-fifths of all senators duly chosen and sworn, or 60 of the 100-member Senate.

About Filibusters and Cloture - Historical Overview, UNITED STATES SENATE website

Filibusters and Cloture in the Senate, CONGRESS.GOV