You will find the differences between the original Dragon Qquest XI and Dragon Quest XI S on this page, of which there are a few. The following is a quick list of the changes/additions to the Definitive Edition, with a more detailed section after it, for those that need a bit more information.

• Can switch between Japanese and English voice acting.
• There is now an orchestral soundtrack.
• Shortcut menu has been added.
• Ability to use the Fun-Size Forge anywhere, via the Shortcuts menu. You can also purchase materials via the forge menu, but not all materials can be purchased.
• Can adjust the battle speed.
• Nose for Treasure added whenever Erik is in party.
• Photo Mode added.
• Other party members appear while exploring (some areas will only have the Hero showing).
• New Draconian Quest options added.
• Can play in 2D Mode.
• New character-specific interludes available between Acts 1 and 2.
• Tickington quests added.
• Outfits now have their own slot, instead of being attached to actual armor. Also, there are new outfits in the game.
• You will get a Horse Hailer item that allows you to summon your horse anywhere, minus places like dungeons and cities.
• New mountable monsters can be found throughout the game.
• Crossbow Targets were removed with the Definitive Edition.
• New trophy list separate from vanilla. Some trophies were added, while others were removed.