A sandbox is a small container filled with sand that children play in. It can also refer to a safe and controlled environment for experimenting, learning, or testing things without any real-life consequences. Full definition
In sandbox games, if / when I beat them, I usually stop there anyway. (gamespot.com)
When you're playing in sandbox mode, there just isn't that much to spice things up beyond continuously expanding your prison till it fills the map. (heypoorplayer.com)
I have a confession to make: I have begun to tire of sandbox games, and, as such, I have never felt more alienated by the games industry. (thatvideogameblog.com)