Piercing

Piercing is identical to the Amazon Skill Pierce; it gives a projectile the chance to pass through a target and continue on to hit a second target for full damage. Bonuses to Piercing stack with the Amazon's Pierce Skill. Numerous Unique Bows/Crossbows have 100% Piercing as a property and you can find +33% piercing on the Unique Belt Razortail as well.

Some skills like Bone Spear or Lightning always pierce through enemies they hit.

Items which add the Piercing characteristic

 * Stormstrike, Short Battle Bow: 25%
 * Razortail, Sharkskin Belt: 33%
 * Gut Siphon, Demon Crossbow: 33%
 * Doomslinger, Repeating Crossbow: 35%
 * Vidala's Rig, set: set: 50%
 * Ichorsting, Crossbow: 50%
 * Warshrike, Winged Knife: 50%
 * Kuko Shakaku, Cedar Bow: 50%
 * , Chu-Ko-Nu: 66%
 * , Ballista: 100%

Diablo III
In Diablo III, many attacks have the piercing characteristic. This allows a single missile or beam to hit all enemies in line (or even in any other trajectory) instead of the first one that happens to be in its path. Most attacks pierce through enemies, regardless of their size and number, but often fail to penetrate solid walls and Waller affix barriers. A piercing attack usually does not diminish in power as it passes through multiple targets.

Barbarian

 * (Pulverize rune, wave)
 * (Sidearm rune, axe)
 * Seismic Slam
 * (Dust Devils rune, tornadoes)
 * (Dust Devils rune, tornadoes)

Crusader

 * (sparks)
 * Blessed Shield (Piercing Shield rune)
 * Phalanx
 * Heaven's Fury (Fires of Heaven rune)
 * Heaven's Fury (Fires of Heaven rune)

Demon Hunter

 * Hungering Arrow
 * Elemental Arrow
 * Evasive Fire
 * Chakram
 * (Overpenetration rune)
 * Rapid Fire (High Velocity rune)
 * Sentry (Impaling Bolt rune)
 * (side guns)
 * (side guns)

Monk

 * Deadly Reach
 * Epiphany (Inner Fire rune)
 * Lashing Tail Kick (Spinning Flame Kick rune)

Necromancer

 * Army of the Dead (Frozen Army rune)

Witch Doctor

 * Plague of Toads (Piercing Toads rune)
 * Firebats
 * Zombie Charger

Wizard

 * Magic Missile (Conflagrate rune)
 * Shock Pulse (Living Lightning rune)
 * Electrocute (Lightning Blast rune)
 * Arcane Orb (Scorch and Frozen Orb runes)
 * Arcane Torrent (Static Discharge rune)
 * Energy Twister
 * Disintegrate
 * Disintegration Wave

Diablo III Items that add / modify Piercing characteristic

 * Dagger of Darts
 * Light of Grace
 * Gyana Na Kashu (only its own fireball)
 * The Ninth Cirri Satchel
 * Red Soul Shard (fireballs)
 * Stalgard's Decimator (only its own axe)
 * Valla's Bequest
 * Valla's Bequest