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); likewise, some objects can only be placed on floor tiles and cannot be placed on terrain. 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 editing is very simplistic compared to later games. Players can raise, lower, and level 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 adjusts the terrain, the grass will die off and will grow back on its own over time. Terraforming costs a small amount of money to complete depending on the extent of the changes done to the terrain.

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 allows 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.

Editing
Terrain editing in The Sims 2 is more variable than in The Sims. 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. Players can level and smooth the terrain in addition to being able to raise and lower it. The Sims 2 also introduces a "Flatten Lot" tool that will instantly level the lot when used. Like in The Sims: Hot Date, ponds can be created anywhere on the terrain.

The Sims 2 also introduces terrain paints that allow for the terrain to be customized. By default, lots 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.

FreeTime introduced the  cheat, which allows the player to adjust the height of terrain in the neighborhood view.

Interactions
Sims can watch clouds during the day and stargaze at night; these interactions can be done on empty terrain located outdoors. Weed can grow on the lawn from a left puddle, which will require to be pulled. In The Sims 2, certain insects may spawn depending of the time on the day: butterflies by day and fireflies by night. Sims and dogs can dig on any piece of empty terrain. If FreeTime is installed, Sims can only birdwatch and collect bugs 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.

Trivia

 * A graphical glitch may occur, where lighting doesn't update for outside objects that had been built around with a room. This can be fixed by reselecting the said object with a hand tool.

Editing
Terrain paints in The Sims 3 are free, unlike previous games.

Interactions
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.