The battlefield, formerly known as the "in-play zone", and even earlier the player's "territory", is a shared game zone where permanents reside after being put there, usually after being cast (for artifacts, creatures, enchantments, and planeswalkers) or played (for lands). Instants and sorceries can't enter the battlefield; those cards are usually put into an owner's graveyard as the last part of such spells resolving.
Prior to the Magic 2010 rules changes, many cards used phrases such as "when [this] comes into play", CIP, "put [something] into play", and so forth. As a result of the changes, cards that referred to something coming into or leaving play were given errata to refer to the battlefield instead.
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 403. Battlefield
- 403.1. Most of the area between the players represents the battlefield. The battlefield starts out empty. Permanents a player controls are normally kept in front of him or her on the battlefield, though there are some cases (such as an Aura attached to another player’s permanent) when a permanent one player controls is kept closer to a different player.
- 403.2. A spell or ability affects and checks only the battlefield unless it specifically mentions a player or another zone.
- 403.3. Permanents exist only on the battlefield. Every object on the battlefield is a permanent. See rule 110, “Permanents.”
- 403.4. Whenever a permanent enters the battlefield, it becomes a new object and has no relationship to any previous permanent represented by the same card, except for the cases listed in rule 400.7. (This is also true for any objects entering any zone.)
- 403.5. Previously, the battlefield was called the “in-play zone.” Cards that were printed with text that contains the phrases “in play,” “from play,” “into play,” or the like are referring to the battlefield. Cards that were printed with that text have received errata in the Oracle card reference.
When an artifact, creature, enchantment, or planeswalker spell resolves, it is put onto the battlefield as a permanent. When a land is played, it is put onto the battlefield as a permanent. Tokens also exist in this zone. See rule 217, “Zones.”