A filibuster occurs when a minority of the Senate declines to vote to invoke what's called "cloture," which sets a deadline to complete debate on a nomination or bill and forces an up or down vote. It takes 60 votes in the 100-member Senate to obtain cloture.
But to Democrats, the vote to deny cloture on Hagel's nomination was a historic, first-ever successful filibuster of a Cabinet nominee and the first time the party in the Senate's minority had ever even tried the move against a defense secretary nominee.
"If this is not a filibuster, I'd like to see what a filibuster was," an angry Majority Leader Harry Reid, D-Nev., said.
But Sen. Lamar Alexander, R-Tenn., shot back that it was Reid who provoked Republicans into the vote by scheduling it too soon. After all, the Armed Services Committee had just approved the nomination two days before and Republicans were still seeking information from Hagel.
"This is a vote by Republicans to say, 'We want more than two days after this nomination comes to the floor to carefully consider it because we have questions,'" Alexander said.
It's in the eye of the beholder as to whether what's going on with the Hagel nomination is a filibuster.
In the Senate, it can take just one senator to force a cloture vote, even if there's clear sentiment for an up-or-down vote. That's part of the reason why Republicans say that just because there's a cloture vote it doesn't mean there's been a filibuster.
On the other hand, several Republicans, like Jim Inhofe of Oklahoma, say they want to require Hagel to cross the 60-vote threshold. That's an argument for calling it a filibuster.
Either way, it's a rare set of circumstances. According to the Senate's historian, Donald Ritchie, just 5 percent of Cabinet nominees have been killed by the Senate or withdrawn when confirmation appeared impossible. And only twice has a Cabinet-level nominee been subject to a cloture vote. It's far more common to filibuster judicial nominees.
Ritchie says that's partly because "judicial appointments are for life and Cabinet appointments rarely last the full length of a presidential administration."