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. However, The Sims 3: Supernatural introduces indoor garden pots. Sim can plant small plants indoor instead of on terrain.

In The Sims 3: Ambitions, Sims can create the miner with the inventing skill. Sim can put it inside their inventory and select any terrain to start drilling.

In The Sims 3: Seasons, there are many outdoor activities that are affected by the weather. This including rake and woohoo in a leaf pile in Fall; make snow angels, builds snowmans and igloos if there is enough snow. Sims can also participate in water balloon or snow ball fights in Summer and Winter.

The Sims 4
The Sims 4 is the first game in the series that does not support terrainforming when release. The Sims 4/Patch 73 adds the terrainforming feature into the game.

The Sims 4: Outdoor Retreat introduces the "Cloudgaze" and "Stargaze" interaction. Sims can select terrain, lying on the ground and Cloudgaze during the day and Stargaze during the night.

In The Sims 4: Seasons, there are many outdoor activities that are affected by the weather. This including rake and woohoo in a leaf pile in Fall; make snow angels, builds snowpals and igloos if there is enough snow. Sims can also participate in water balloon or snow ball fights in Summer and Winter.