We step across an invisible boundary where those crops give way to bananas.

On his return, he found that the flock had not left the invisible boundary.

Worlds are constrained by their size, the size of the world being the amount of build-able land before you reach an invisible boundary which objects may not pass outside of.

I like to give her as much freedom as possible within those invisible boundaries.

However, they are often confined to one part of the game world by invisible boundaries.