A standalone title is a complete, self-contained game that does not require prior knowledge of a series to understand its story, characters, or mechanics. It has no mandatory prequels or sequels.
Uncharted: Lost Legacy, Infamous First Light, Dishonored: Death of the Outsider – sold separately, playable without base game, but share engine/assets/lore. Marketed as standalone, technically DLC-scale.
Bottom line: If you can buy it, put it in, and get the full intended experience with zero prerequisites → it's standalone.