Code: Blue denotes an emergency medical situation requiring immediate attention. A Code Blue is initiated when a patient is unresponsive, pulseless, or not breathing: i.e. the patient needs cardiopulmonary resuscitation (CPR). Once there is a Code Blue, a team of doctors and nurses, often a designated "code-team", will rush to the patient taking life-saving measures. The team uses a crash cart which contains important aids such as a defibrillator, intubation equipment, suction, oxygen, and an I.V. setup to stabilize the patient.

Not to be confused with a Code: Brown, which is ER-speak for when a patient doesn't make it to the bathroom or bedpan in time.

Disclaimer: I'm not a doctor or nurse. I've never participated in a Code Blue. Corrections, elaborations are welcome.