Open world
Also said as Free roam
One large continuous place you can cross as you wish, doing things in nearly any order. It promises that if you can see it, you can go there.
Freedom costs the designer control over pacing, so the world itself has to pull you along. Too many map icons and the world turns into a list of chores. With too few, it is a long empty walk.
Say it
“No icons on the map at the start. Put a landmark you can see from anywhere every 300 metres, so that the player navigates by looking and sets their own next stop.”
Where you’ve seen it
Grand Theft Auto III (2001) made it a mainstream expectation. Assassin's Creed's viewpoints fill your map with icons once you climb them. The towers in The Legend of Zelda: Breath of the Wild (2017) show the land and no icons, so you go by what catches your eye.
Not to be confused with linear
A linear game chooses what happens next. An open world lets you choose, and has to make every choice interesting, wherever you go.