a low area where the land is saturated with water
"The migrating birds flew over the wetland, a low area where the land was permanently saturated with water."
Land that is covered mostly with water, with occasional marshy and soggy areas.
"The local wetland was covered mostly by standing water, though visitors still encountered several marshy and soggy patches along the edges."
In plain English: A wetland is an area of land that stays soggy with water for most of the year, creating habitats full of plants and animals adapted to living near or underwater.
"The local wildlife sanctuary is built on a large wetland that protects many migrating birds."
Usage: Wetlands are ecosystems saturated with water where plants like reeds and cattails thrive in muddy or swampy conditions. Use this term to describe natural habitats such as swamps, bogs, and marshes rather than simply any damp ground.
The word wetland is a straightforward combination of the words wet and land that has been used to describe low-lying areas covered by water since its formation in English. No specific historical shift occurred because the meaning remains exactly what it was when first coined.