* Pokémon Legends: Arceus: This is the full name of a specific game released in 2022. It's an open-world action RPG set in the ancient Hisui region (which will later become the Sinnoh region from Pokémon Diamond and Pearl). The game focuses on the early days of the Pokémon world and the creation of the Pokédex.
* The "Legends" Series: There is not an official "Legends" series in the Pokémon franchise. Pokémon Legends: Arceus is a standalone title.
* "Legends" in General: Some fans might use "Pokémon Legends" to refer to any game that deals with the lore or history of the Pokémon world, including games like Pokémon Mystery Dungeon: Explorers of Time/Darkness/Sky or even certain episodes of the anime.
So, to be clear, "Pokémon Legends" usually refers to Pokémon Legends: Arceus, a game set in the ancient past of the Pokémon world.