Knock back

From TF2 Wiki

Knock back is a side effect of some weapons that pushes your character away from the source of the knock back. In Team Fortress 2, knock back varies from very slight (only preventing you from moving forward), to extreme (launching you several feet into the air). Knock back can be useful strategically, such as when juggling opponents

Sources of Knock Back

There are several sources of knock back in the game ranging from weapons and buildings, to the environment itself.

Weapons with knock back	Severity of knock back
The Soldier's Rocket launcher	Causes players to be launched through the air
The Demoman's Sticky grenade launcher	Causes players to be launched through the air
The Engineer's Sentry gun bullets	Pushes players away from the sentry, can launch airborne targets high into the sky
The Engineer's Sentry gun rockets	Causes players to be knocked backwards
The Spy's Butterfly knife Backstab	Causes players to be hurled forwards (Only affects ragdolls)
The Pyro's deflection ability	Causes players and explosives to be sent backwards, rockets reverse direction
Objects with knock back	Severity of knock back
The CP_Well trains	Has the ability to hurl characters across the map (Only affects ragdolls and debris)