From Bulbapedia, the community-driven Pokémon encyclopedia.
Greenfield (Japanese: グリーンフィールド Greenfield) is a town in the Pokémon world that only appeared in the anime. It is the setting for the third movie.
Greenfield is described, in the anime, as one of the most beautiful locations in Johto. It is almost completely crystallized during the course of Spell of the Unown. It contains a Pokémon Center as well as a play park (where Lisa and Ash battle in the third movie) and the Hale mansion, home to Spencer Hale, Molly Hale and Mrs Hale (Molly's mother and Spencer's wife). Its meadow-like surrounding is the main attraction of Greenfield. According to the anime maps, it is located just southeast of Goldenrod City.
Greenfield while under the Unown spell.
In Greenfield, Molly Hale discovers some tiles that her dad left behind when he was kidnapped by the Unown and sent into an alternate dimension. The tiles contain the Unown, who proceed to make Molly's wishes come true, including turning all of Greenfield into crystal.