Which expansion introduced the 'Azerite Armor' system?

The Battle for Azeroth expansion introduced the Azerite Armor system. Released in 2018, Battle for Azeroth replaced Legion's class-specific artifact weapons with a new progression system centered on Azerite and the Heart of Azeroth.

Azerite Armor consisted of special helmets, shoulders, and chest pieces. These items contained concentric rings of traits that players unlocked as the Heart of Azeroth gained levels. The traits could improve particular spells, grant defensive effects, or add bonuses suited to a class and specialization.

The system was tied to the story of Azeroth's wounded world-soul. After the events of Legion, the planet began producing Azerite, a powerful substance that both the Alliance and Horde sought to control. The player character received the Heart of Azeroth from Magni Bronzebeard, allowing Azerite power to be collected and used.

A common mix-up is confusing Azerite Armor with Legion's artifact weapons or the later systems in Shadowlands and Dragonflight. Azerite Armor was specific to Battle for Azeroth and occupied only three equipment slots.