Knockback is an effect of an Attack that pushes the character hit away in the direction of the Attack. For Melee Attacks, this is typically the direction the attacking character is facing, while for Ranged Attacks, this is typically the direction-of-travel of the projectile.
Knockback velocity is often constant throughout the game, but there can be modifiers applied to a given Attack’s Knockback based on stat changes, Status Effects, or the nature of the Attack itself.