Regrowth means “Return target card from your graveyard to your hand.”

Each mode uses a card name as shorthand for its text. Growth Charm’s name is still Growth Charm, no matter how it grows.

Rampant Growth means “Search your library for a basic land card and put that card onto the battlefield tapped. Then shuffle your library.”

Giant Growth means “Target creature gets +3/+3 until end of turn.”