From Bulbapedia, the community-driven Pokémon encyclopedia.
A Z-Move (Japanese: Ｚワザ Z-Move) is a special type of move in which a Trainer and their Pokémon's wishes combine to unleash an attack comprising the full power of both. These moves are very powerful, but they can only be used once per battle.
There are two different kinds of Z-Moves: Z-Moves that power up a move depending on its type and can be used by any Pokémon, and special Z-Moves that can only be used by specific Pokémon. The Z-Move that a Pokémon can perform depends on the Z-Crystal it holds. There are 18 different Z-Crystals that correspond to each of the 18 types, and several others that correspond to specific Pokémon. If a Pokémon is holding a Z-Crystal corresponding to one of the types, then it must also know a move of that type in order to perform that Z-Move.
A Z-Ring is also required in order to perform Z-Moves. A Pokémon can only use a Z-Move if its Trainer is wearing a Z-Ring with a Z-Crystal set in it that matches the Z-Crystal the Pokémon holds.
Z-Moves were introduced in Generation VII.
List of Z-Moves
For each type
For specific Pokémon
In other languages