Exigent Circumstances: Definition from Answers.com Mobile

Emergency situations or conditions which the law recognizes as excusing compliance with some procedural requirement or recognition of another’s property or other interests. Most commonly used to refer to the variety of contexts in which a valid search and seizure may be conducted without a warrant. If the police action must be taken on a “now or never” basis to preserve evidence, it may be reasonable to permit a seizure without obtaining prior judicial approval. 413 U.S. 496, 505.
Exigent circumstances may be found when substantial risk of harm to others or the police would exist if police were to delay a search until a warrant could be obtained. 627 F. 2d 906, 909.
The mobility of a motor vehicle has been held in itself to create an exigent circumstance. In every instance where a search or arrest warrant has been dispensed with on grounds of exigency, probable cause must be present to justify the intrusion. 453 U.S. 454, 457.