'Battle cry' was introduced in the Scars of Mirrodin block in the Mirrodin Besieged set.

702.88. Battle Cry

• 702.88a - Battle cry is a triggered ability. "Battle cry" means "Whenever this creature attacks, each other attacking creature gets +1/+0 until end of turn."
• 702.88b - If a creature has multiple instances of battle cry, each triggers separately.

Translation:

• 702.88a - Battle cry happens in response to the creature being declared attacking.
• 702.88b - The abilities stack so a creature with two instances of Battle cry will get +1/+0 twice.

Notes from Mirrodin Besieged FAQ:

• The +1/+0 bonus applies to each other attacking creature, but not to the creature with battle cry itself.
• The +1/+0 bonus remains until the end of the turn, even if the creature with battle cry leaves the battlefield or stops being an attacking creature.
• If a creature with battle cry attacks more than once in a turn (due to an additional combat phase being created), its battle cry ability will trigger each time. The results are cumulative.
• In Two-Headed Giant and other multiplayer variants where players share a turn, attacking creatures controlled by a teammate also receive the +1/+0 bonus.