Rule 601.1 · Casting Spells
Previously, the action of casting a spell, or casting a card as a spell, was referred to on cards as “playing” that spell or that card. Cards that were printed with that text have received errata in the Oracle card reference so they now refer to “casting” that spell or that card.
Subrules
Some effects still refer to “playing” a card. “Playing a card” means playing that card as a land or casting that card as a spell, whichever is appropriate.