Knockback

is a game mechanic that pushes a player or mob away when they are hit.

Increasing Knockback

 * enchantment (applied to Swords)
 * enchantment (applied to Bows)
 * - ability applies large Knockback to all mobs in a radius.
 * - ability applies large Knockback to all mobs in a radius.

Knockback Resistance

 * - provides immunity to Knockback from mobs.
 * (Pet Item) - provides immunity to Knockback.
 * - ability provides immunity to Knockback.
 * Rotten Armor - Gain additional  Knockback resistance to arrows.
 * Tank - Class Passive: Defensive Stance Grants immunity to Knockback from mobs.

True Knockback
was a type of that ignored all anti-knockback items except Slime Hat. It was similar to, but applied to Knockback. True Knockback was removed alongside Knockback fix.

Sources:
 * Non-Dungeon bosses:
 * Ender Dragon's Fireball attacks
 * Revenant Horror and Sven Packmaster's anti-cheese attacks
 * Emerald Slimes
 * Bal's launch attack
 * Dungeon bosses:
 * Bonzo's Balloon barrage, as well as the attack in his Blood miniboss version
 * Necron's wither skull attack ignores all Knockback resistance
 * The Undead Warrior summoned by Scarf when it hits a Player with Warrior's Charge
 * The Professor's Guardians' attack in which it slams itself against the ground, pushing the player back in a force similar to the Revenant Horror and Sven Packmaster's anti-cheese attacks
 * Dungeon mobs:
 * Some of Thorn's minions deal true Knockback
 * Lost Adventurer's "Dragon's Breath" Ability
 * Withermancer's homing skull deals heavy true Knockback upon implosion
 * Defend Jerry's Workshop Magma Cubes' attacks
 * NOTE: Some mobs that attack extremely fast, such as Terracottas, are strong enough to cause lag and bypass Anti-Knockback. Turtle Shelmet will NOT prevent this sort of knockback.

True Knockback Resistance
Mounts gave you True Knockback Resistance while riding, but removed the Immunity Ticks (10 ticks, around 0.5s), that could cause you to die easily. Unlike, True Knockback Resistant items also applied for regular Knockback sources.
 * - Knockback immunity while riding.
 * - Knockback immunity while riding.
 * - Knockback immunity while riding.
 * - Knockback immunity while riding.
 * - Knockback immunity while morphed.

Tips

 * The Enchantment is generally undesired as pushing enemies away often increases the time to kill them. In most cases, the default Knockback applied by weapons is considered sufficient.