Center: The vertical axis of a tropical cyclone, usually defined by the location of minimum wind or minimum pressure (different from the eye). The cyclone center position can vary with altitude.

Eye: The low pressure center of a tropical cyclone. Winds are normally calm and sometimes the sky clears. Eyes can range from five to more than 50 miles across.

Eye Wall: The ring of thunderstorms that surrounds a storm's eye. The heaviest rain, strongest winds and greatest violence are normally in the eye wall.

Hurricane: A tropical cyclone with winds of 74 mph or more. Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line.

Knot: A measure of speed. It is one nautical mile per hour. A nautical mile is one minute of one degree of latitude and is slightly longer than the ordinary, or statute, mile used in the United States.

Landfall: The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur.

Major Hurricane: A hurricane with highest winds of 111 mph or more.

North Atlantic Basin (Atlantic Basin): The Atlantic Ocean north of the equator, the Caribbean Sea, and the Gulf of Mexico.

Storm Surge: The dome of water that builds up as a hurricane moves over water. As this water comes ashore with the storm, it causes flooding that is usually a hurricane's biggest killer. A strong storm surge may be up to 50 miles wide and 20 feet high.

Tropical Cyclone: A warm-core, non-frontal, synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center.

Tropical Depression: A tropical cyclone with maximum sustained winds near the surface of less than 39 mph. Tropical depressions are listed only with a number, not a name.

Tropical Storm: A tropical cyclone with winds of 39 to 74 mph. In most of the world, a storm is given a name when it reaches tropical storm intensity.

Tropical Disturbance: Often the earliest stages of a tropical cyclone. Normally an organized area of thunderstorms that forms in the tropics and persists for more than 24 hours. Low pressure might form at the surface, but winds around remain below 30 mph.

Tropical Wave: A kink or bend in the normally straight flow of surface air in the tropics which forms a low pressure trough, or pressure boundary, and showers and thunderstorms. Can develop into a tropical cyclone.

Saffir/Simpson Hurricane Scale: A scale ranging from one to five, based on intensity of the hurricane. This can be used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane.

Subtropical Cyclone: A low pressure system that develops in subtropical waters and initially has non-tropical features but does have some element of a tropical cyclone's cloud structure.

Typhoon: A hurricane in the north Pacific west of the International Date Line. Sometimes the word is used to refer to any tropical cyclone, no matter what its wind speed.