Video Games Encyclopedia

A walking simulator, shortened walking sim, is an adventure game that consists primarily of movement and environmental interaction. Walking sims sometimes include puzzle elements, and generally do not have combat mechanics or traditional win/lose scenarios. While these video game elements originated in the 1980s, the term began to be used pejoratively to refer to new games as walking simulators in the late 2000s, notably with Dear Esther. The term was eventually used less pejoratively and adopted by gamers, while still being negatively perceived by some game developers and retaining negative gameplay connotation. Other descriptors have been commonly used for games of such style, including empathy, narrative, and exploration game. Such games are often a hallmark of art games, but some mainstream games have been described as having walking simulator elements.
The central elements of walking simulators are controversial due to purported lack of challenge, and discontent of such games became common in the mid-to-late 2010s among “hardcore” gamers. However, the artistic aspects and emphasis on decision-making and morality are appreciated and remain popular across other video games.

Reference: Wikipedia