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
In The Sims, terrain was generally very simplistic. Players can raise or lower the terrain as a basic form of terraforming, although Sims will typically have difficulty using objects placed on a slope. In addition, players cannot build walls near or on sloped tiles, nor can they add floor tiles of any kind. When the player raises or lowers the terrain, the grass will die off and will grow back on its own over time.

With Hot Date, players could add water to the terrain (previously, this was only possible through the use of cheats) that allowed for natural ponds and rivers. Hot Date also introduced a Grass tool that allowed the player to add or remove grass manually.

Unlike later games, Sims do not have the option to cloud watch or stargaze on empty terrain. If Vacation is installed, Sims can build snowmen or sandcastles on terrain, depending on which area of Vacation Island they were on. They can also rent a metal detector and will roam around the lot, uncovering buried items on empty terrain.

The Sims 2
Terrain editing was expanded in The Sims 2. Editing the terrain no longer kills off grass, and floor tiles can be placed on sloped terrain. In addition, while The Sims was limited to basic hills, slopes, and valleys, The Sims 2 allows for much more extreme slopes and creations to be made.

Sims can watch clouds during the day and stargaze at night; these interactions can be done on empty terrain located outdoors. Dogs might dig holes in residential lot's yard.If Bon Voyage is installed, Sims can dig for treasure on any piece of empty terrain. If FreeTime is installed, Sims can only birdwatch, collect bugs, and catch butterflies (during daytime) or fireflies (during nighttime) on empty terrain—the interactions will not be available on any outdoor floor tiles. In the winter, snow-related interactions, such as making snowmen and having snowball fights, are only available on empty terrain.

Terrain paint
By default, lots in The Sims 2 will take on the terrain type of the neighborhood it is placed in. For example, a lot placed in a desert-themed neighborhood will have desert terrain paint by default. Players can override the default terrain and paint their own using the Terrain Brush tool in build mode. Only exposed terrain (that is, terrain not covered by floor tiles) can be painted. Terrain paint can be removed later on by selecting the tool and ctrl-clicking on the terrain paint.

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.