David Patten urges skepticism of the term:
There is no dispute about the dire situation in Iraq. Insurgents, militias, terrorists, and death squads are killing civilians at an alarming rate. Security forces are unreliable, and the Iraqi government is not meeting the needs of the people. Iraq is in a worse state than U.S. policymakers expected it would be three years ago.
However, it does not follow that Iraq is in a civil war. While the government is weak, there is no political force presenting it with a serious challenge. Iraq is, indeed, an unstable nation, but there is little danger of regime change, the ultimate purpose of a civil war. The armed groups most likely to participate in an eventual civil war lack both the capacity and the will to enter into such a conflict in earnest at the present time.
This does not mean that violence will decline; quite the contrary, as the referendum on the future status of the disputed city of Kirkuk nears, violence may increase. Nor does the central government appear able to consolidate power in the short term. Its inability to provide security and basic services will lead local officials, including unelected leaders of religious factions, to assume more power. But, in the long term, the central government will survive and take on a more significant role in keeping Iraq unified. For U.S. and coalition policymakers, assisting Iraq's transition to democracy will require patience, diplomacy, and ingenuity.
However, unfounded concerns over a civil war erupting could prompt an overreaction from U.S. policymakers. Should they conclude that Iraq is in a civil wareven if they base their determination on political expediency and no clear criteriathe most likely response would be a demand for withdrawal. A premature withdrawal of coalition forces could motivate the Sunni Arab insurgency to unify behind a political program; Sunni Arab civilians would likely lose any remaining confidence in the security forces, and many more would flee their homes. The Jaysh al-Mahdi undeterred would expand its influence and become the government's rival for the people's loyalty. Premature withdrawal could lead to a self-fulfilling prophecy, creating the conditions for a civil war that do not currently exist.
Matt Yglesias responds here.