The fetch, often called the fetch length, is the length of water over which a given wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

 has blown. It is used in geography

Geography

Geography is the science that studies the lands, features, inhabitants, and phenomena of Earth. A literal translation would be "to describe or write about the Earth". The first person to use the word "geography" was Eratosthenes...

 and meteorology

Meteorology

Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

 and is usually associated with coastal erosion

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

. It plays a large part in longshore drift

Longshore drift

Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...

 as well.
Fetch length along with the wind speed (or strength) determines the size of waves

Ocean surface wave

In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Waves in the...

 produced. The longer the fetch length and the faster the wind speed, the larger and stronger the wave will be. For example, the winds which travel from the East Coast of the United States

East Coast of the United States

The East Coast of the United States, also known as the Eastern Seaboard, refers to the easternmost coastal states in the United States, which touch the Atlantic Ocean and stretch up to Canada. The term includes the U.S...

 and hit the west coast of Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

would have an extremely large fetch and would produce very large waves if the wind speed was also high. The fetch length determines the power and energy of the wave. Additionally, if the winds are blowing in the same direction during the wave's lifetime, the wave will in turn be stronger. If a fetch is very large, then the wave will be very large and vice versa. The fetch is related to the orbit of the wave. The longer the wind drags along the sea the more energy the wave will have which will make the wave a destructive wave.