ÜberCharge

An ÜberCharge, commonly referred to as an "über", is a game-mechanic specific to the Medic in regular gameplay. By healing other players with his medi gun, the Medic is able to fill up an ÜberCharge meter. When full, the charge can be activated and the player being healed will be granted a buff, the type of which depends on the selected medi gun:

• The stock Medi Gun yields invulnerability to damage (except certain hazards, such as pits).
• The Kritzkrieg yields guaranteed critical hits, even for weapons that can't normally deal (random) critical hits.

Although only the Medic can charge and use ÜberCharges in regular gameplay, this mechanic is specifically tied to the weapon; if a class other than the Medic is given a medi gun, they too can utilise ÜberCharges.