Sandbox
Also said as Software toy, Creative mode
A game that gives you tools and rules but no set goal. It promises that the only limit is your idea, and what you make is why you play.
Players supply their own reasons, so it never runs out of content, and it creates the stories people share. But an empty box scares many players, who need a nudge to start.
Say it
“Ship a sandbox mode: unlimited materials, no enemies and no score. Every material should react with at least two others, so that people find things we did not plan.”
Where you’ve seen it
Minecraft (2011) is the great example: its creative mode gives you every block, with no goal, and people have built whole cities in it. Garry's Mod (2006) did the same with physics props and had no objectives of its own.
Not to be confused with open world
An open world lets you go anywhere. A sandbox lets you change anything. Plenty of open worlds cannot be changed at all.