A special set of endgame armor, similar to other MMORPG's. Normally an endgame set of armor/weapons that increase in rarity as they increase in "Tier" Example: Tier 2 is harder to earn than Tier 1.
In DCUO, once you reach Level 30, you must earn this "tier" armor. There are 4 armors: Tier 1, the first and easiest armor, Tier 2, the harder armor to earn, and PvP, which is earned from PvP (Arena or Legends). Tier 1 and Tier 2 are usually used for Alerts in hard mode, Duos, and Raids.
The hardest gear to acquire is the Tier 3 & 4 raiding PvE gear. This can be acquired in one of the wings of your faction HQ by the means of Marks of Triumph, which can be earned through Challenge Solo Missions, Duos, Alerts and Raids. The Prime Battleground, Gates of Tartarus, Batcave: Inner Sanctum alert and missions from The Hand of Fate earn you Marks of War which are used to purchase Tier 4 PvE gear.