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. According to Professor Magnolia, the Pokémon distorts space to change its size while affecting the world around them. This transformation has another special variant known as Gigantamax.

https://bulbapedia.bulbagarden.net/wiki/Dynamax

See also:

• Macro
• Micro