Delineation is the affixing of precise boundaries. Although federal agencies, states, and textbook authors vary in the way in which they define wetlands, in general terms, Wetlands are lands on which water covers the soil or is present either at or near the surface of the soil or within the root zone, all year or for varying periods of time during the year, including during the growing season. There also are wetlands that lack hydric soils and hyrdrophytic vegetation, but support other organisms indicative of recurrent saturation. So that seems fairly straightforward - what's the problem. Water depth - wetlands can range from wet to dry, some are continually under water, others have seasonal surface water, some are saturated but never have water at the surface.
Wetlands are often transitional - between dry uplands and deep water habitats.
This idea of scale can complicate things. We often like to define based on scale. Location is highly variable. Some are in lowlands, some on sideslopes, some are in mountains, some by oceans. Again the natural variability is large.