From Bulbapedia, the community-driven Pokémon encyclopedia.
Dynamax (Japanese: ダイマックス Daimax) is a temporary transformation affecting Pokémon that was introduced in Generation VIII. Trainers who obtain a Dynamax Band are able to Dynamax their Pokémon. Dynamaxing increases a Pokémon's size drastically, as well as changing the moves of the Pokémon and increasing their stats in battle, but can only be used once during a battle, and lasts only for three turns. In addition, it can only be performed in certain areas, such as special arenas in Gyms.
All moves that a Pokémon knows will turn into Max Moves, which are more powerful than regular moves. The Max Moves that are learned are determined by the moves that the Pokémon knows.
Wild Pokémon in a Max Raid Battle start out as Dynamax Pokémon, and do not return to their original state during the battle. However, the Trainers' Pokémon do not start out as Dynamax Pokémon, and only one of the Trainers' Pokémon can Dynamax during the battle, and only for three turns.
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