Player character and others may stagger or otherwise be caused to lose steady footing. In first person games this likely involves the view and aim swaying somewhat uncontrollably.
The first video game about Staggering was released on November 11, 2011.
Knockback does not imply any loss in controls. Staggering implies either complete loss of controls for a moment or them working in somewhat different manner that prevent normal operation (or simply make it exceptionally hard to function normally).
This may overlap with stunning if player loses complete control during the stagger though it would be preferable to tag only stunning in such cases.