What is the definition of "exigent circumstances" in the context of search and seizure cases?

Exigent circumstances are defined as "circumstances that would cause a reasonable person to believe that entry (or other relevant prompt action) was necessary to prevent physical harm to the officers and other persons, the destruction of relevant evidence, the escape of the suspects or some other consequence improperly frustrating legitimate law enforcement efforts." United States v. McConney, 728 F.2d 1195, 1199 (9th Cir.) (en banc), cert. denied, 469 U.S. 824 (1984).