A walking simulator is exactly as the name suggests. It’s a game that involves a lot of walking, and walking, and some more walking.
These games are also called environmental narratives which is a nicer term.
They don’t JUST contain walking, although that’s the main portion of them, but a lot of walking sims allow you to look at pieces of the environment for backstory, or contain puzzles to progress.
Some examples of well-known walking simulators are:
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