Terrain

Terrain is an area of land, usually referred for just a lot but not limited in that scope. Players can modify it by terraforming or painting it through build mode tools. Elements can affect the said terrain, such as snow and rain. Certain objects, especially outdoor plants, can only be place on empty terrain (unless cheats are used). There are also certain interactions and activities limited to empty terrain that can't be performed anywhere else, including but not limited to catching insects or bugs, collecting, doing various snow activities (making snow angels, building snowmen and having snowball fights), having water balloon fights, watching stars, etc.

The Sims 3
The collecting system in The Sims 3 comes with collectible spawners. These spawners only spawn collectibles on terrain.

Sims can only grow plants on terrain, unless they use a planter bowl.

In Ambitions, Sims can build the miner with the inventing skill, which allows them to start drilling on any vacant terrain.

Terrain was heavily expanded in Seasons, as many seasonal activities, such as raking leaves, making snowmen, and constructing igloos, can only be done on empty terrain.

The Sims 4
Terraforming did not come with the initial release of The Sims 4, but was reintroduced with patch 73.

Outdoor Retreat reintroduced cloudgazing and stargazing into the game. Like The Sims 2, Sims can only do this on empty terrain.

Much like The Sims 2: Seasons and The Sims 3: Seasons, The Sims 4: Seasons introduces seasonal activities that can only be done on empty terrain.