From Bulbapedia, the community-driven Pokémon encyclopedia.

Anime physics is a fanon term used to describe all the unusual, impossible, and sometimes absurd moments in the anime where real-world physics do not seem to apply. At times, the term can also be used to describe when an attack works differently in the anime than it would in the games. Some common examples are unusual feats of strength and agility, surviving lethal attacks, and the magic satchel theory, sometimes called hammerspace (the ability to hold innumerable items in one bag).

Anime physics are often caused by several factors. Inconsistencies in animation, such as objects changing size in-between scenes or even single video frames, are usually the result of unintentional mistakes on the animators' part. Some mistakes, such as characters frequently having different heights throughout an episode or show (without specific reason), are simply the result of a lack of skill or care on the animators' part.

Deviations from standard laws defined in the real world are common in some genres of animation, in this case, the Pokémon anime. Characters may sustain damage that normally would result in mortal injuries or immediate death, but instead are simply left soot-covered (in the case of explosions or incineration), disheveled, or fatigued. Characters may not age, whether or not time does actually pass within the fictional universe. In the Pokémon universe, main characters often mention weeks, months, or even a year to have passed, but themselves do not age. Ash Ketchum, who at his debut was confirmed to be 10 years old, remains at the age of 10 to this day, despite the fact that episodes occasionally state weeks to have passed since the previous one.

Examples

Loose physics

Some phenomena that appear in the anime (as well as the entire Pokémon canon) are impossible in real-world physics.

Magic satchels

Contents of Cilan's backpack.

Move and type effectiveness errors