What is the parliamentary procedure allowing the US senate to override a standing rule by a simple majority?

In the United States Senate, the nuclear option is a parliamentary procedure that allows the Senate to override a standing rule by a simple majority, rather than the two-thirds supermajority normally required to amend Senate rules.

The nuclear option was first invoked in November 2013, when a Senate Democratic majority led by Harry Reid used the procedure to eliminate the 60-vote rule for presidential nominations, other than nominations to the Supreme Court. The nuclear option is a last-resort; it is an emergency way for the majority party in the Senate to overcome obstruction by the minority.

The option allows the US Senate to act with a simple majority of 51 votes. Under the previous rules, 60 votes were needed to foil any attempt by the minority party to block a vote by use of the filibuster.