What is a hacked Pokémon?

Excuse me, could you please elaborate on the concept of a "hacked Pokémon"? I'm somewhat unfamiliar with the term, and I'm curious about its implications in the world of Pokémon gaming. Is it a reference to a Pokémon that has been illegally modified or manipulated in some way? Are there any specific methods used to create such a Pokémon, and what are the potential consequences or risks associated with owning or using one?