Fetch is the length of sea surface over which wind acts without obstacles. Together with wind speed and duration, it determines the energy and maximum height of waves. Greater fetch produces larger waves with longer periods.
Examples
The North Atlantic has fetches of thousands of kilometres, generating 10–15 m waves in storms. The Mediterranean, enclosed with maximum fetches of ~1,000 km, produces smaller waves (rarely >6 m). Channels like the Strait of Gibraltar limit fetch but accelerate wind.