An open world game gives the player a large continuous environment that can be explored freely in many directions, with objectives available in flexible order. Fast travel and story gates usually exist, but the world itself is not divided into a strict level sequence.
The contrast is with linear design, where the player moves down a corridor of set pieces in the designer order. An open world instead presents a map and a set of possibilities: story missions, side activities, collectibles, and unmarked places that exist only to be found. Grand Theft Auto III in 2001 showed the commercial scale of a fully explorable city, and The Elder Scrolls and later The Legend of Zelda Breath of the Wild showed how differently the same freedom can be paced.
Freedom needs structure to stay interesting. Designers guide without forcing, using landmarks on the horizon, roads that curve toward points of interest, and difficulty that rises in certain regions. Too little guidance produces an empty map with icons. Too much produces a checklist that happens to be outdoors. The best open worlds reward ignoring the objective marker entirely.
Open world is a structure, not a genre by itself. It hosts RPGs, shooters, racing games, and survival games, and it scales from a single dense city block to maps that take real hours to cross on foot. Size alone is not the achievement; density of meaningful choices per minute of travel is.
In play"Ignore the main quest for a while and just explore, because the open world is full of optional towns."