ÜberCharge (supercharge or overcharge in German) is an ability operated by a Medic on his healing target from his medicine gun by pressing the alternative-fire key. The Medic and the patient receives eight seconds of invulnerability, with the exception of a Pyro's Compression Blast or other sources of Knockback. ÜberCharges are often performed on Heavies, due to their mini-gun's heavy gunpower that takes out enemies in seconds, or less than a second if they are unlucky to get in the way.
When a Medic switches to his secondary slot, he will get a HUD on his current ÜberCharge percentage, as well a progress meter being filled in as the percentage complete is increasing. The highest rate the Medic can heal is at a Setup time in Payload or Attack/Defend game mode.