Fetch is a term used in oceanography to describe the area of ocean or lake surface over which the wind blows in an essentially constant direction, thus generating waves. Fetch length, which is the horizontal distance over which wave-generating winds blow, is also used as a synonym for fetch. In an enclosed body of water, fetch is also defined as the distance between the points of minimum and maximum water-surface elevation. Fetch length, along with wind speed and duration, determines the size of waves produced. If the wind direction is constant, the longer the fetch and the greater the wind speed, the more wind energy is transferred to the water surface and the larger the resulting sea state will be. Wave height is affected by wind speed, wind duration, and fetch, which is the distance over water that the wind blows in a single direction. If wind speed is slow, only small waves result, regardless of wind duration or fetch. If the wind speed is great but it only blows for a few minutes, no large waves will result even if the wind speed is strong and fetch is unlimited. The longer the wind blows and the greater the distance it covers, thus the greater the fetch, the greater the wave height.

In summary, fetch is the area of water over which wind blows in a constant direction, generating waves. Fetch length, wind speed, and duration determine the size of waves produced. The longer the wind blows and the greater the distance it covers, the greater the fetch and the greater the wave height.