Definition:
A hurricane warning indicates that hurricane winds of 74 mph (64 knots) and higher, or a combination of dangerously high water and rough seas, are expected to impact a specified coastal area. When a hurricane warning is announced, hurricane conditions are considered imminent and may begin immediately, or at least within the next 12 to 24 hours. When a warning is announced, it is of utmost importance that precautionary measures are taken for protection of life and property.the following conditions are occurring or imminent. A hurricane watch is announced when hurricane conditions are posssible within 36 hours.
Further reading about hurricane warning/watches:
Marine Weather Advisories, Watches and Warnings