When the presiding officer of the U.S. Senate disregards a rule or precedent.
This most commonly refers to an effort by the Senate to end a filibuster by a simple majority, even though rules specify that ending a filibuster requires the consent of at least 60 senators.
An opinion written by Vice President Richard Nixon in 1957 concluded that the U.S. Constitution grants the presiding officer the authority to override Senate rules in this way. If a majority vote to uphold the presiding officer, his interpretation of the rules becomes a precedent.
Senator Trent Lott (R-MS) first called the option “nuclear” in March 2003, using the metaphor of a nuclear strike to suggest it might provoke retaliation by the minority party.