A Senate hold is how a senator informally signals his objection to a bill or nomination.
Most congressional actions clear parliamentary hurdles by “unanimous consent” of the Senate, so a senator who intends to object to such procedures can, effectively, hold up the action. He may announce his intentions publicly or, more frequently, inform his party leader and place a “secret hold” on an action. Holds have become more common since the 1970s, when the Senate began using many more unanimous consent agreements to advance a greater volume of legislation, and opponents have suggested many changes to reform or abolish the practice.
The most recent challenges to this custom include a 2010 letter in which 69 senators pledged not to place holds and a 2011 resolution declaring that, in the case of secret holds, either a senator’s identity is revealed after two days or the hold is assigned to the party leader. The latter of these reforms has been easily circumvented by the tag-team hold.